Gemara

Mishnah: If one covered the blood and it became exposed, he need not cover it again. If the wind (blew earth and) covered it, he must cover it.

Rabah bar bar Chanah: This is only if it became uncovered. If not, he is exempt.

Question: Even if it becomes uncovered, once the Mitzvah was Nidcheh (pushed off), it should not return!

Answer (Rav Papa): This teaches that Ein Dichuy b'Mitzvos.

Question: Reish Lakish asked 'if a Nochri bowed to a tree, may one use a Lulav from it for the Mitzvah?' Why didn't he learn from the Mishnah?

Answer: Reish Lakish knew that Ein Dichuy b'Mitzvos. He was unsure if the Tana was certain about this (and says Ein Dichuy even to be lenient), or if he was in doubt (and is stringent to require covering in case the Halachah is that Ein Dichuy). His question is not resolved.

Chulin 83b: Kisuy ha'Dam does not apply to Kodshim, because earth must be put also below. If we were Mevatel the earth to (stay on) the Mizbe'ach, the Mizbe'ach would be too big. If the earth will be removed later, it would be considered a Chatzitzah (interruption) between the blood and the Mizbe'ach!

87a (Mishnah): If the wind covered it (he must cover it.)

Question: Why is this different than if the blood was absorbed into the ground?

(Beraisa): If the blood was absorbed into the ground, one must cover it.

Answer: That is when it is noticeable (one can see a discoloration where the blood was absorbed).

(Mishnah): If the blood became mixed with water, if it still looks like blood, it must be covered. If it was mixed with blood (exempt from Kisuy, e.g.) of a Behemah, we view the (exempt blood) as if it was the same amount of water;

Rishonim

Rif (Chulin 29a): If the blood was absorbed into the ground and it is not noticeable, it is as if the wind covered it, and he is exempt.

Rambam (Hilchos Shechitah 14:7): If he covered the blood and it became exposed, he need not cover ita. If the wind covered the blood, he need not cover it. If it became exposed again, after the wind covered it, he must cover it.

Rambam (9): If he slaughtered and the blood was absorbed into the ground, if it is noticeable, he must cover it. If it is not noticeable, it is as if the wind covered it, and he is exempt.

Rosh (Chulin 6:8): If the wind covered the blood and it became exposed again, he must cover it, for Ein Dichuy b'Mitzvos.

Divrei Chamudos (28): If Dam Chayah was covered by Dam Behemah (and if the Dam Behemah were water, the mixture would not look like blood), one need not scrape off the Dam Behemah in order to cover the Dam Chayah. This is like when wind covered the blood. Even though we exempt when wind covered it only if it was with things Kosher for covering, we view the Dam Behemah as if it were water (and it need not be covered).

Tosfos (83b DH Tzarich): It is a Mitzvah to put earth on top, for if he covered the blood and it became exposed, he need not cover it, but if the wind covered the blood and it became exposed again, he must cover it. There is no Mitzvah to put earth underneath. If there is earth underneath, it is as if he put it.

Poskim

Shulchan Aruch (YD 28:10): If the blood was absorbed into the ground, if it is noticeable, he must cover it.

Shulchan Aruch (11): If the wind covered the blood, he is exempt.

Simlah Chadashah (12, in Sefer Tevu'os Shor, and cited by Tosefes Merubah in Shulchan Aruch ha'Shalem): This is only if the wind covered it with things Kosher for covering, and there was also something Kosher for Kisuy below. If not, one must reveal it, scrape it and put earth above and below without a Berachah.

Tevu'os Shor (16): Presumably, the Seifa (in which the wind covered it) is like the Reisha, in which a person covered it. It is reasonable that the blood is Nidcheh from covering only if it was covered by things Kosher for covering. Divrei Chamudos considered this to be obvious. I am unsure. The Gemara asked from the Beraisa that obligates when the blood was absorbed, and answered that this is when it is noticeable. What was the question? One may not cover with hard land! We require loose earth, like Rashi said (83b DH Afar). "From the Afar (earth) that will be in the Karka (land) of the Mishkan" connotes that earth and land are not the same! Nidah 8b connotes that when the earth is not loose, this is called virgin land, but Nidah 13a, Bava Basra 19b and 105b connote that Stam land is not loose earth. Don't say that the Gemara gave a better answer, that if the blood is noticeable, even if the earth is loose he must cover it again. The Rambam says that if the blood was absorbed, it is as if the wind covered it. I.e. they are the same. With difficulty, we can say that we require loose earth because it mixes with the blood and the blood is absorbed in it, like Rashi says. If so, land is Pasul because the blood does not mix with and get absorbed in it. We require something that will immediately absorb the blood. However, if one waited until it was absorbed inland, it is a proper covering. However, Semag disqualifies land because we require "Afar", i.e. something that crumbles and one can count the granules. Rather, the Mishnah applies whether the wind covered it with things Kosher or Pasul for covering. Even if the Mitzvah was not done, now it is covered. I am unsure. One should be stringent to cover in any case of Safek, but without blessing. If blood was absorbed in the ground and it is noticeable, one should scrape it and cover it (above and below). Perhaps the land below is considered like earth (b'Di'eved, after the blood was absorbed), but in any case one may bless for covering on top. If it was covered above and below with things Pasul for covering, he scrapes the blood and covers again, but without a Berachah, for perhaps there was Dichuy, and we do not bless when there was Dichuy. If it was not absorbed below, even if one covered it with earth above, he scrapes the blood and covers without a Berachah. When one slaughtered a Behemah on top of Dam Chayah, the Poskim say that one need not scrape and cover. Perhaps this is because one cannot separate the bloods. When it is possible, one must scrape and cover.

Yad Avraham: Clearly, Tosfos is sure that even if the wind covered with things Kosher for covering, one must cover it again if it became uncovered.

Shulchan Aruch (ibid): If it became exposed again, he must cover it.

R. Akiva Eiger: He covers it again without a Berachah. Regarding a Kvi (a Safek Chayah, Safek Behemah), he would be exempt, for it is a Sefek Sefekah. Perhaps it is a Behemah, and perhaps Dichuy applies to Mitzvos. However, the Sefekos did not arise at the same time. The Safek of covering arose once it was slaughtered, and after the wind covered it the Safek of Dichuy arose. Perhaps the Sefekos must arise at once (to be lenient about a Sefek Sefekah. Drush v'Chidush (Berachos 2a) says that they need not come at once if both Sefekos are about what is the Halachah.)

Shulchan Aruch (ibid): If he himself covered the blood and it became exposed again, he is exempt.

Simlah Chadashah (13, cited by Tosefes Merubah): One should not cover in a place where it is prone to get uncovered, and one may not uncover it for the blood, but one need not guard it from being uncovered.

Toras Chayim (87a DH Kisahu): The Mishnah connotes that l'Chatchilah, one may not uncover it. Why do people slaughter chickens in the street? Everyone walks there. Surely it will be uncovered! Rather, the Mishnah discusses the usual case, but l'Chatchilah one may uncover it. Since one need not cover it again if it was uncovered, this shows that it suffices to cover it for a moment.

Tevu'os Shor (18): I say that even if one may uncover it, this is only if he covered it properly, in a place where it could remain covered. What will surely get uncovered is as if it is uncovered. The Ro'oh (Bedek ha'Bayis Daf 19) says that this is why one must scrape blood off the knife to cover it. The knife is not a covering, for it will be removed. The Rashba disagreed, but not about this principle. We asked (83b) 'if one will not be Mevatel the earth, it will be a Chatzitzah.' Why didn't we say that if the earth will be removed, it is not a covering? Perhaps the Hava Amina was to put so much earth below that afterwards it can all be removed and the blood would not be visible at all. This does not apply to a knife. It is difficult to say that the Mishnah taught a typical case, rather, than a bigger Chidush! Rather, even if one covered it properly, he may not uncover it for the blood, for it looks like he covered it (to save it) for this (when he will need it). Some people slaughter in the house in places that are swept. Perhaps the blood, or at least some of it, will be absorbed before it is swept. One may not slaughter where all of it will be uncovered. The Pri Chodosh was lenient; I do not understand this. He cited the Rashba, but overlooked that the Rashba is stringent!