Esau was watching. The Targum Pseudo-Jonathan on Genesis 28:6 lingers on what he noticed: not only that Isaac blessed Jacob, but that Isaac sent Jacob to Padan Aram with a very specific instruction — do not take a wife from the daughters of Canaan.
For Esau, this was a revelation with teeth. He had already married two Hittite women who made life bitter for Isaac and Rebekah (Genesis 26:34–35). He had assumed the marriages were merely his own taste. Now he heard, through the crack in the tent door, that the covenant came with a prohibition he had already violated.
The Aramaic paraphrase draws attention to the moment of moral clarity Esau receives too late. The blessing had a fence around it. The fence was: the mothers of the covenant must come from the covenant. Sarah had been brought from Aram. Rebekah had been brought from Aram. Now Jacob would walk the same road Rebekah's servant had walked a generation before (Genesis 24:4).
Esau's response in the next verses — marrying a daughter of Ishmael — reads less like repentance and more like a man trying to patch a hole after the water has already poured through. He cannot unmarry his Canaanite wives. He can only add a cousin from the outer edge of Abraham's family and hope it counts.
The takeaway: the rules of the covenant are not invisible. They are written into the way the patriarchs actually lived. Esau saw them only when the blessing passed him.