The Torah introduces the chapter of Yitzchak's blessings to his children by recording Yitzchak's loss of vision: "When Yitzchak was old and his eyes were too dim to see, he called his older son Esav… " (27:1). On one level, this information is necessary for us to understand how Yaakov managed to trick his father and disguise himself as Esav. However, the Abarbanel interprets this verse as the basis for not only Yitzchak's inability to identify the recipient of his blessing, but also his preference of Esav over Yaakov. Simply put, Yitzchak's "vision," or judgment, was blinded by his love for Esav. (Raboteinu Ba'alei Ha-Tosafot explain similarly.) The Abarbanel felt compelled to explain the verse in this manner because of the immediately preceding verse, which records Yitzchak's response to Esav's marriage to Hittite woman: "They were a source of bitterness to Yitzchak and Rivka" (26:35). After his disappointment upon seeing Esav marry a Canaanite woman, how could Yitzchak then immediately prepare to give him a special blessing? Abarbanel therefore explains that Yitzchak was simply blinded.
Though the Abarbanel bases this clouded judgment on Yitzchak's love and affection for his favorite son, one well known Midrash in Bereishit Rabba may give rise to a different approach. The Midrash relates that when Yitzchak sat bound upon the altar as his father Avraham prepared to slaughter him in fulfillment of God's command, the heavenly angels began crying. Their tears descended straight into Yitzchak's eyes, causing the onset of blindness as he reached old age. A slightly different version appears in other sources (the Riva - one of the Tosafists - , for example, cites this version from the Pesikta), by which Yitzchak simply looked straight through the heavens to the Divine Throne, causing him eventual blindness. What does all this mean?
Perhaps the Midrash points to the akeida episode as the ultimate expression of absolute truth and sincerity. It represents the most extreme ideal of performing God's will without allowing any external considerations to interfere. Not only did God's command at the akeida require both father and son to overcome man's most basic and natural feelings (in Yitzchak's case, self preservation; in Avraham's, a parent's affection for his child), but it also contradicted every possible line of reasoning and defied the most elementary common sense. The fulfillment of this command thus reflected the pinnacle of obedience - performing God's will and setting aside all questions and other considerations.
This ideal of truth, perhaps, allowed Esav to mislead his father and earn his affection and respect. As we know, Chazal claim that Esav intentionally fooled his father into believing that he meticulously observed detailed mitzvot and faithfully adhered to the legacy of Avraham. This duplicity runs in diametric opposition the Yitzchak's quality of truth expressed atop Mount Moriah as he was bound upon the altar. The image of Yitzchak gazing straight to the Divine Throne perhaps refers to his ability to see only what is right and ignore everything external. He could not conceive of trickery and deceit. Thus, it was the experience of the akeida that "blinded" his judgment concerning his son and led him to attempt to bestow a special blessing upon Esav.