Why doesn't Isaac just take back the blessing once he realizes that he's blessed the wrong son – that is, Jacob rather than Esau?
The question refers to the incident when Isaac tells Esau, his first-born twin, to make him his favorite meal so he can bless him before he dies (which, incidentally, isn't for another 40 or so years). But Rebekah, Isaac's wife, overhears this plan and convinces Jacob to pretend to be Esau in order to get the blessing from his father. Rebekah's plan works in this respect, and Isaac unintentionally blesses Jacob, the younger son (Gen 27).
Nahum M. Sarna explains in the JPS Torah Commentary that, although Isaac is completely rattled by the mistake, he realizes "the blessing he has uttered is beyond recall. According to the conception of the times … the destiny that has been solemnly conferred upon his younger son is irreversible." For this reason, "Esau does not ask his father to rescind the blessing, only to bless him as well" (193-4).
W. Sibley Towner confirms in his commentary, Genesis, that blessings, once uttered, can't be revoked:
Scholars have long claimed that in the minds of ancient peoples generally and also in ancient Israel, language was thought to be dynamistic. Once a word was said, it did its work independently from the blesser and even of God. A blessing or a curse, once released, could not be retracted or even changed. (206)
There were cultures that believed that words had a magical power all their own. But I wonder if Abraham and his descendents believed that a blessing could work independently of God. The way Sarna explains Isaac's blessing offers a different perspective:
We see … that the source and sanction of the blessing is not man but God. Isaac summons from the very depths of his own soul all the vitality and energy at his command in order to invoke God's blessing upon his son. He communicates the blessing to his offspring by virtue of his own special relationship with God and by dint of his power and authority as patriarch. (190)
This view of God as the originator of the blessing would lead to the conclusion that such a blessing would work according to God's plan, not independently. What's more, a blessing from God would be impossible to reverse.
Whether or not people, specifically the Patriarchs, believed that the power of a blessing was in the words themselves or came from God, they did believe that it couldn't be taken back. What was said was said and became fulfilled.
Sarna, Nahum. JPS Torah Commentary: Genesis. Philadelphia: Jewish Publication Society, 1989.
Towner, W. Sibley. Genesis. Westminster Bible Companion. Louisville: Westminster John Knox P, 2001.