Gemara

(Rav Papi): One blesses '... Leva'er (to get rid of) Chametz';

(Rav Papa): He blesses 'Al Bi'ur Chametz'.

All agree that Leva'er connotes [to get rid of in] the future. They argue about Al Bi'ur;

Rav Papi [does not use this phrase because he] holds that it connotes the past. Rav Papa holds that it connotes the future.

Question (Beraisa): One who circumcises blesses '... v'Tzivanu Al ha'Milah.' (This shows that it connotes the future!)

Answer: There, one cannot bless 'Lamul', for this implies that he himself must circumcise. Really, someone else could circumcise!

(Rav Yehudah): On all Mitzvos, we bless before doing them.

(Bei Rav): The only exception is immersion.

This is because beforehand, he is not ready.

Berachos 12a: Perhaps [in the Mikdash] they did not bless Yotzer Ohr because it was not yet time, and they would say it later in its time [when daylight comes]!

51a (Rav Chisda): If one ate or drank without blessing, he should bless [now, before eating more].

(Ravina): Therefore, even if he finished his meal, he should bless:

(Beraisa): After immersion, he blesses "... Asher Kidshanu b'Mitzvosav v'Tzivanu Al ha'Tevilah."

Rejection: That is not a proof. There, he could not bless before immersing;

Here, he could have blessed. Since he did not, perhaps he is Nidcheh (pushed off) from blessing afterwards!

Rishonim

Rambam (Hilchos Berachos 6:2): Anyone who washes his hands, whether to eat, for Keri'as Shema or Tefilah, blesses beforehand Al Netilas Yadayim.

Rambam (Hilchos Milah 3:1): One who circumcises someone else blesses beforehand 'Al ha'Milah.' If he circumcises his son, he blesses 'Lamul Es ha'Ben.'

Rosh (1:10): We bless on all Mitzvos before doing them. The only exception is immersion, because beforehand, he is not ready. Rabanan say that this refers only to Tevilah of a convert. He cannot bless beforehand Asher Kidshanu. Anyone else blesses beforehand. We hold that a Nidah may separate Chalah, even though she must bless. We learned that if a Zav saw Keri (a seminal emission), or semen left a Nidah, they must immerse [before learning Torah or praying - Berachos 26a]. If not for the semen, they could learn and pray without immersing. The Ri says that we do not protest against women who bless after Tevilah. Since a convert cannot bless beforehand, we do not distinguish. This reason applies also to Netilas Yadayim. The custom is to bless after washing, since in the morning his hands are dirty, and he cannot bless until after washing. Therefore, the custom is to do so even when washing for a meal. Also, since he blesses before drying, also this is the Mitzvah of washing, like it says in Sotah 4b, that one who eats bread without drying his hands is like one who eats Tamei bread.

Ran (3b DH v'Kashya): A Mitzvah that one must fulfill himself, all agree that one must bless on it with a "Lamed" [at the beginning, e.g. Leshev"]. A Mitzvah that one can do through a Shali'ach, all agree that the Shali'ach blesses 'Al'. The Amora'im argue about one who does such Mitzvos himself. If Al connotes also the future, it is better to bless Al, to teach that one can do the Mitzvah through another. One blesses Al Netilas Lulav, even though he must take it himself, for he was already Yotzei once he picked it up.

Poskim

Shulchan Aruch (OC 158:1): One blesses before washing his hands, for we bless on all Mitzvos before doing them. The custom is not to bless until after washing, for sometimes his hands are not clean. Therefore we bless after rubbing his hands, for they are already clean before the second pouring.

Magen Avraham (15): Even though one need not dry when he pours a full Revi'is at once, we do not distinguish. (I.e. before drying is always considered before the Mitzvah.)

Mishnah Berurah (39): We bless just before doing the Mitzvah.

Mishnah Berurah (40): E.g. sometimes he just left a bathroom, or touched a covered part of the body,

Mishnah Berurah (41): The custom is to bless after washing, before drying.

Rema: One can bless on them also before drying, since also drying is part of the Mitzvah. This is called before the Mitzvah. If one forgot to bless until after drying them, he blesses afterwards.

Beis Yosef (DH u'Mah she'Chasav v'Od): Tosfos and the Rosh say that since he blesses before drying [also this is the Mitzvah of washing]. However, they do not connote that one must bless specifically after washing. Rather, they give reasons why the custom is to bless after washing. R. Yerucham saw his Rebbeyim bless after rubbing the hands. All agree that one must bless before drying.

Taz (12): This is difficult, for sometimes one need not dry, like it says in Sa'if 13 (e.g. one who immerses his hands, or pours a Revi'is each time)! However, the first reason (sometimes his hands are not clean) suffices. The Rema connotes that one may not bless Al Netilas Yadayim after ha'Motzi. The Yam Shel Shlomo (Chulin 8:39) says that if one forgot, he may bless before ha'Motzi, but not afterwards. I do not understand his proof. It seems to me that after the Mitzvah is finished, i.e. drying, one may not bless. One blesses just before doing the Mitzvah. Surely, for a Mitzvah that extends over time, if one forgot beforehand, he may bless while engaging in it, but not after finishing it. This is why the Tur wrote to bless before drying, and did not permit after drying. I bring a proof from 167:8. If one remembered after the meal that he did not bless ha'Motzi, he does not bless. Do not say that here is different, for he still did not eat. (Perhaps he means that there remained a Mitzvah to eat with Tahor hands - PF.) This is wrong. He could eat with Tahor hands without touching [the food]. If he put his hands in gloves, he need not wash later (Beis Yosef Siman 164). Also, if so, he should bless "who commanded us to eat with Tahor hands", for the Mitzvah of washing already finished! Do not ask from the Rema (172:1), who says that [if one put liquid into his mouth without a Berachah,] he swallows it and blesses. There is different, for he remembered while they were still in his mouth, like the Rosh says. I bring a proof from there, for the Gemara said 'once [the Berachah] was Nidcheh, it was Nidcheh.' Also here, once it was Nidcheh from before drying, it was totally Nidcheh.

Magen Avraham (16): For other Berachos, one does not bless afterwards, for what was Nir'eh (proper for a Mitzvah) and Nidcheh does not become Nir'eh again [even if the reason it was Nidcheh went away]. Here is different. Since at the beginning he was not proper to bless [since sometimes the hands are filthy], he may bless afterwards. See 432:1. (If one did not bless before beginning Bedikas Chametz, if he did not yet finish, he blesses now (Rema). If he finished, he blesses before burning, or while burning (Magen Avraham).) Sefer ha'Tanya says that if one is unsure whether or not he blessed, he does not bless again. R. Natrunai Ga'on says that if one erred and did not bless and ate bread, he does not bless.

Kaf ha'Chayim (86): If one is unsure whether or not he blessed, he should think the Berachah in his heart.

Hagahos Ashri (Chulin 1:2, citing Or Zaru'a): One who slaughters must bless beforehand like R. Yochanan. We are not concerned lest the Shechitah be invalid. If he did not bless before Shechitah, or any Mitzvah, he can bless afterwards, and he was Yotzei, like was explained in Berachos.

Hagahos Ashri (Berachos 1:13): If one blessed after the Mitzvah, he was Yotzei. However, regarding a meal, one may not benefit from the world without a Berachah. Since he ate, and it is time for a Berachah Acharonah, since it was Nidcheh, it is Nidcheh.

Gra (DH v'Im): Hagahos Ashri says that b'Di'eved one can bless after the Mitzvah. The Rambam disagrees based on 51a. Hagahos Ashri distinguishes between Birkas ha'Mitzvos and Berachos on food. The Gemara connotes unlike this. It brings a proof from Tevilah to Berachos on food! Hagahos Ashri learns from Berachos 12a, which says that they would bless Yotzer Ohr after Keri'as Shema. This is not conclusive. Birkos Keri'as Shema were not enacted for [the Mitzvah of] Keri'as Shema, like the Rashba and Tosfos (7b DH b'Idna) say. However, according to the first reason (he could not bless beforehand), one may bless, like in Berachos 51a. The same applies to Lulav. See Magen Avraham 651:11. (If one did not bless on the Lulav before taking it, he blesses afterwards.) The Rema is lenient according to both reasons.

Mishnah Berurah (44): Here, we do not say 'once it was Nidcheh, it was Nidcheh', for initially it was not proper to bless, since sometimes the hands are dirty. Therefore, even if he already dried, he may bless. However, the Poskim say not to bless Al Netilas Yadayim if he already said ha'Motzi. The Taz is stringent not to bless even after drying. In practice we do not follow this, for many Acharonim agree with the Rema. Even so, l'Chatchilah one must be very careful about this. Some people say "Se'u Yedeichem..." [after washing] slowly, and they do not say Al Netilas Yadayim until after drying. This is improper.

Kaf ha'Chayim (83): The custom is like the Rema. Also the Ari Zal says to bless between washing and drying.

Kaf ha'Chayim (85): Since Poskim argue about this, if one did not bless before drying, he should scratch his head or touched a covered part of the body. If one touched a covered part of the body during the meal [so he must wash again], the Taz and Magen Avraham bring from Maharshal not to bless on it. There, he already blessed. Here, he did not yet bless, and it is before the meal, perhaps the Maharshal would agree. The Mishbetzos Zahav and Ben Ish say so.

Bi'ur Halachah (DH Mevarech): The Shulchan Aruch connotes that one may bless before washing if his hands are clean. The Tur said not to. The Pri Megadim said that this is before before the Mitzvah, and not [simply] before the Mitzvah. However, R. Chananel says to bless before washing, and the Rambam connotes like this, so we do not protest against one who does so.