Or HaChaim on Genesis 27:33/אור החיים על בראשית כ״ז:ל״ג

1 ויחרד יצחק. Isaac trembled. Isaac experienced contradictory feelings; he trembled because he had been tricked. At the same time he was aware that he had blessed Jacob by saying that anyone cursing him should himself be cursed. When he understood that the person upon whom he had bestowed the blessing was his son Jacob he blessed him because he realised that the fragrance that emanated from Jacob's clothing indicated his moral superiority over Esau. This prompted Isaac to confirm his blessing. He may also have been afraid to bring a curse upon himself if he were to revoke the blessing.