What do we learn from "Mevarech Re'ehu b'Kol Gadol ba'Boker Hashkem Kelalah Techashev Lo"?
Erchin 16a: This refers to one who received nice hospitality and publicly blesses his host who toiled so much for him. People hear this, and take his money. (Rashi 16a - powerful people hear of his wealth, and take it by force. Alternatively, guests inundate him until his money is consumed.)
Rashi citing Erchin 1 : One who praises his friend every day, it is converted to a curse. People say that he pardons his money, and he is wealthy, and all borrow from him, and the kingdom contests him, to take from him.
Rashi citing Tanchuma Balak 24: This refers to Bil'am. He blessed Yisrael in a loud voice 2 - "va'Yisa Meshalo
Our text in Erchin is somewhat different, as Rashi himself explained there (refer to 27:14:1:1). (PF)
Tanchuma: Hashem made his voice strong, and the entire world heard, so the nations will hear that he blesses.
If the Brachah is not at a time of Ratzon, why should it be considered a curse? The Gemara implies that it is because he puts his friend's honor before Hashem's. Shalom is forbidden, for it is a name of Hashem (OC 89:2).
Even if he does not deserve a Brachah, why should it be considered a curse? (PF)