At the end of his life, Yitzchak decides to give a blessing of prosperity and family dominance to his oldest son Esav. His wife Rivka hears of his plan and concocts a charade to enable the younger son Yaakov to surreptitiously 'steal' the blessing from the blind Yitzchak. There are numerous difficulties with this episode. Why does Yitzchak want to bless the morally and spiritually deficient Esav? Why doesn't Rivka share with her husband her clear prophetic knowledge that Esav was destined to be subservient to Yaakov? Finally, how can a blessing intended for Esav end up being fulfilled in Yaakov?
Unlike Avraham, Yitzchak was unclear which of his two sons would take over after his death. He recognized that Yaakov's moral and spiritual commitment surpassed Esav's, but in the end he decided that Esav's first-born status tipped the scales. It is likely that his intense love for Esav blinded him to the truth. The verse that introduces this episode, "...Yitzchak had become old and his eyes dimmed..." should be interpreted both literally and figuratively. However, in order to provide Esav the opportunity for spiritual merit, Yitzchak directed him to prepare a meal from scratch in order to at least demonstrate his commitment to honoring parents.
Rivka had received a clear prophecy before the twins were born: "Two nations are in your womb...the elder shall serve the younger." The reason she did not share this prophecy with Yitzchak initially may have been due to the fact that she had sought out the advice which resulted in the prophecy without Yitzchak's permission. She also may have deferred to his higher spiritual and prophetic level. However, to share the prophecy with him now could actually be counterproductive. Still blinded by his love for Esav, Yitzchak could end up blessing Yaakov begrudgingly or withholding the blessing from both sons, leaving everything in G-d's hands. Rivka wanted Yitzchak to give Yaakov the blessing voluntarily and with a full heart.
In the end, Yaakov successfully passes himself off as Esav and receives a full-hearted blessing from his father. How can a blessing in error take effect? Abarbanel explains that a human being, even a prophet, is only a conduit for a blessing, a blessing which ultimately comes from G-d. The blessing is essentially a prayer, a petition for a desired result. This can be explained metaphorically: A farmer intends to plant barley but accidentally plants wheat instead. His intention has no effect on the outcome. Only wheat will grow. Yitzchak is only a conduit for the will of G-d. The blessing will only 'grow' in Yaakov, just like only wheat will grow in the field.