Why was Issac blind? The answer to this question is a wonderful illustration of Rashi's way with midrashim, and also an indictment of how our children are taught in the typical Yeshiva.
The average Torah tyke (and alas, some Torah tykes have long beards and wrinkles) (Torah-tyke is my phrase. You can' t use it) the average Torah tyke will tell you that our forefather Isaac lost his sight because the tears of angels fell into his eyes when he was tied to the alter. This is problematic: (1) Angels aren't physical, so how could they produce physical tears?; and (2) why would their tears be corrosive to Isaac's sight? I could cry in your eyes, for example, without doing any damage.
Explaining this midrash is beyond the scope of this post (also, I haven't completely worked it out yet) but if you look in Rashi, you'll see that he gives two other reasons for Isaac's failing sight:
(1) The smoke from the sacrifices offered to idols by Esav's Hittie wives;
(2) Done by God, for the purpose of delivering the Blessing to Jacob.
You might also, notice that some editions put the bit about the tears in brackets, suggesting that it does not appear in every variant reading of Rashi.
Also, if you refer to Rashi's source material, you will see that there are two other explanations, which Rashi does not use:
(A) Abimelech said to Sarah (Gen 20:16): ולשרה אמר הנה נתתי אלף כסף לאחיך הנה הוא לך כסות עינים לכל אשר אתך ואת כל ונכחת׃ (unto Sarah he said, Behold, I have given thy brother a thousand pieces of silver: behold, he is to thee a covering of the eyes, unto all that are with thee, and with all other: thus she was reproved.) The midrash construes this as a curse of blindness, which fell on Isaac, Sarah's child
(B) The blindness resulted from looking at his evil son Esav (the author of this aggadah held that looking at bad people degrades the eyes) (the author of this aggadah also held Esav was evil)
So what gives?
Rashi's comment, in which he mentions just two (or perhaps three) of the available explanations serves to address the juxtaposition of the information. We're told Issac was blind immediately before the blessing story begins. Why? Because the blindness was manufactured by God for the purpose of the blessing (see 2) We're also told about Esav's Hittie wives, and the foul odor their sacrifices produced, immediately before we're told about Isaac's blindness. Why? Because the smoke caused his eyes to fail (see 1 above)
(Those who argue that the the story about the tears was originally included, and not added by a later scribe, say that it is there to explain why the smoke affected Issac, and not Rivka, ie: his eyes had already been weakened by his experience on the alter)
The other two explanations (A and B above) can not be used to address a textual issue, and this Rashi excludes them. Unfortunately for the Jewish people, our children are rarely -if ever- given this sort of insight into Rashi's method and thinking. Instead, they are fed the tale of the tears as if that was the sole authentic Torah True explanation, when, in fact, there are four others. Instead, they grow up thinking the Rashi is a midrash anthology, which he is not.