Yitzchak loved Eiav, and at first glance it seems Yitzchak had the wool pulled over his eyes, with the deceitful questions of Eisav such as how to tithe salt, but clearly Yitzckak was aware of the true nature of Eisav as when Yaacov came dressed up in Eisav's clothing he almost gave himself away by mentioning Hashem's name. This led Yitzchak to suspect that the son in front of him was not in fact Eisav, as Eisav would never mention Shem Hashem.

If so, then why was Yitzchak so keen on giving Eisav the bracha?

The Kerem Hatzvi beautifully explains that Yitzchak was indeed aware of the show Eisav was putting on and went along with it, as at the very least, Eisav was trying to impress his father the tzaddik. Eisav still had a sensitivity to the Torah values of his father, even if he himself wasn't on that level. Yitzchak is relieved that his son isn't trampling and degrading the Torah values so totally that he doesn't even care what his father thinks, and so he plays along. As long as there's still a connection, there is hope.