What did Yitzchak mean when he said "Birchasecha" (your Berachah)?
Seforno: He meant the Berachah that was fitting for Esav - matters concerning Olam ha'Zeh (Ha'amek Davar - even without Torah and Avodah). 1
Malbim: Wealth, power and authority are not proper to be called Berachos - unless they are used for spirituality. Your Berachah was to supply Yaakov's income and protect him, so he can engage in Torah. I intended that Yaakov will be poor, and rely on you for these. He took them for himself, and does not need you. Now, there is no benefit to give them to you - they will be for your detriment!
What was Yitzchak's answer why he cannot bless Esav as well?
Malbim: Wealth, power and authority are not proper to be called Berachos - unless they are used for spirituality. Yitzchak thought that he will give them to Esav, and Yaakov will be poor, and rely on Esav for income and protection, so he can engage in Torah. Now that I gave these to Yaakov himself, there is no benefit to give them to Esav; they will be for his detriment!
Ohr ha'Chayim (to 27:37): Yitzchak was making excuses not to bless him, for he understood that Esav is Yaakov's enemy, and he should be cursed - "Orerecha Arur" (27:29).
QUESTIONS ON RASHI
Rashi writes (to 27:33) that the Berachos belonged to Yaakov. How could Yitzchak tell Esav that Yaakov obtained the Berachos fraudulently?
Rashi, Targum Onkelos and Targum Yonasan: "b'Mirmah" means with Chochmah (astutely), implying a trickery that was commendable. 1
Ramban: Refer to 27:33:5:3.
See Sifsei Chachamim.
Rashi writes: Yitzchak told Esav that Yaakov took the Berachos via "Chochmah." Even if Yitzchak considers this commendable, still it is Lashon ha'Ra, for it caused Esav to want to kill him!
Ha'amek Davar: Indeed, initially Yitzchak did not say who it was, due to Leshon ha'Ra, and lest he cause hatred. Only after Esav said (27:34) "Barcheni Gam Ani," i.e. your Berachah took effect, for it was your son 1 [Yaakov], Yitzchak admitted.
Malbim: Only the coming to take the Berachos was with cunning. The Berachos themselves belong to Yaakov; wealth and authority go with Chochmah - "bi'Semolah Osher v'Chavod," "Bi Melachim Yimlochu" (Mishlei 3:16, 8:15). Also had I blessed you, it would have been for him!
He realized now that HaSh-m wanted Yaakov to get the Berachos, to be financially independent and not be tied to Esav. He told Esav in order to distance him from Yaakov. He did not fully realize Esav's nature, that he would want to kill Yaakov, or he held 2 that a Rasha cannot harm a total Tzadik. Also, it was prone to become known. Perhaps it would be a Chilul HaSh-m if Esav thought (and perhaps he would publicize) that only he himself was blessed, and we find that Yaakov prospers more (PF).