The Torah tells us that Yitzchak was blind in his old age. According to one explanation provided by Rashi, G-d caused this to happen "in order to enable Yaakov to take the blessings."
Now, to say that blindness is debilitating is an understatement, as Rashi comments, "because Yitzchak's eyes had become dim and he was confined to his house... he was as though dead." Nevertheless, because Yitzchak planned to bestow the greatest blessings upon Eisav, not knowing the full extent of his wickedness, G-d caused him to become blind in order for Yaakov to successfully disguise himself as Eisav and receive the blessings instead.
Consider this: Yitzchak was 123 years old at the time of the blessings, and he lived until the age of 180. So for 57 years Yitzchak suffered from blindness, confined to his house, "as though dead," all because he was unaware of Eisav's wickedness and therefore desired to bless him. Was this really necessary? Why didn't G-d simply inform Yitzchak that Eisav wasn't worthy of the blessings, just as He communicated many other things to him?
Moreover, Yitzchak already had suspicions about Eisav. He disapproved of Eisav's wives and he was aware that Eisav did not have the praise of G-d at the tip of his tongue. So why did G-d not reveal the whole truth about Eisav instead of causing Yitzchak to be blind for the last 57 years of his life? From here we see the extent to which one must be willing to go in order to avoid speaking negatively about another Jew. G-d deemed it preferable to cause Yitzchak to be blind for over five decades rather than to tell him lashon hara about the wicked Eisav!
If this is true concerning Eisav, how much more careful must we be when speaking about our fellow Jews, the most sinful of which are certainly not nearly as wicked as Eisav was!
(the Rebbe, Likutei Sichot, Vol. 15)
Beneath Eisav’s Surface
Yitzchak was not entirely oblivious to Eisav's unrefined character and conduct, yet he intended to bestow upon him the most powerful blessings. Surely, his desire to bless Eisav was not based only on what he did not know about him, but also on what he did see in him.
The Torah relates that Yitzchak devoted a great deal of energy to digging wells, restoring old wells and searching for and developing new sources of fresh water.
Chassidus explains that this was not a meaningless detail in Yitzchak's life, but a reflection of his distinctive nature, and his unique path in the service of G-d.
Where others saw dirt and gravel, Yitzchak dug deeply to reveal springs of life-giving water that ran beneath the earth. This means, in spiritual terms, that Yitzchak devoted himself to refining the unrefined, unearthing and cultivating the potential within all creation to recognize its G-dly source.
Accordingly, we can understand why Yitzchak desired to bless Eisav. Despite Eisav's disreputable behavior, Yitzchak had the ability to see the great spiritual potential associated with Eisav's soul. Yitzchak therefore hoped that his powerful blessings would successfully excavate the spiritual treasures that lay hidden in his child. Nevertheless, despite Eisav's immense potential, his actual lowly state was such that the blessings would not have had the desired effect on him. Rivka therefore deemed it necessary for Yaakov to receive the blessings instead of him.
Today, there are no Eisavs, but like Yitzchak, who labored to reveal the connection to G-d buried within the wicked Eisav, we must endeavor to draw near those who seem distant from the life-giving waters of the Torah. With toil and effort, we can reveal within them their innate connection to the Source of Life.
(the Rebbe, Likutei Sichot, Vol. 15)
Adapted from the teachings of the Rebbe, reprinted from Ohr HaChassidus