Seforno: Yitzchak [initially] suspected that Ya?akov was worthy of a curse, like Ya?akov himself stated in Pasuk 12, "Veheveisi Alai Kelalah". The Gemara states in B?rachos, 31b that someone who suspects his friend of something that he did not do must not only appease him, but bless him as well - like Eli blessed Chanah.'

Ohr ha'Chayim - before feeling him, Yitzchak thought that Ya?akov is Mesa'te'a (feigning to be Eisav). Even a Chacham's resentment in his heart can bring a curse (Bava Metzia 84a). Yitzchak blessed him (when he concluded that it is Eisav) to remove the effect of his prior resentment.

Malbim: Yitzchak's eating was the preparation for the B?rachah. Just like offering a Korban puts Shechinah on the one who offered it, and through Kohanim eating, the owners get Kaparah, so Yitzchak's eating gave Divine power to Ya?akov. Chazal said that the goats were a commemoration of Chagigah and Pesach.

Ha'amek Davar: He concluded that it is Eisav; he changed his voice and mentioned Hashem's name to make it easier for Yitzchak to get Ru'ach ha'Kodesh - "B?rachosChol ha'Makom asher Azkir es Shemi..." (Sh?mos 20:21). He blessed him for this.