Genesis 27:37 - Too late to change.

And Isaac answered and said unto Esau, Behold, I have made him thy lord, and all his brethren have I given to him for servants; and with corn and wine have I sustained him: and what shall I do now unto thee, my son?

Isaac initiated the blessing but it's God that fulfills the blessing (The source of all true blessings). Apparently, once the blessing is in God's hands, the blessing is irrevocable. Therefore, there was nothing Isaac could do to change what God had already ordained for Jacob (Genesis 27:28).