What caused Yitzchak to go blind?
Rashbam: Old age. 1
Rashi #1: It was due to smoke of Ketores that Esav's wives burned to idolatry. 2
Targum Yonasan, Riva: When he was bound on the Mizbe'ach by the Akeidah, he saw the Divine Throne. 5
Rashi #3: To enable Yaakov to come and take the Berachos.
Seforno: Because he failed to rebuke Esav. 6
Ba'al ha'Turim, Da'as Zekenim, Hadar Zekenim #1: Bribery blinds. His judgment was flawed because Esav brought him animals.
Megilah 15a, Moshav Zekenim, Hadar Zekenim #2: It was due to Avimelech's curse of Sarah ("Kesus Einayim" - 20:16). Refer to 20:16:2:5.
Yoma 28b: It is because he looked at the face of a Rasha (Esav). Also refer to 27:1:1.3:1.
Ha'amek Davar: It was so he will think that he is about to die, and give the Berachah.
Refer also to 24:1:1:7* for another reason as to why the Torah inserts "va'Yehi Ki Zaken Yitzchak." Ha'amek Davar - it was not due to old age. Almost a third of his life remained!
Rosh, Da'as Zekenim, Moshav Zekenim: If so, also Rivkah should have gone blind! If bone hits bone, it makes no impression, but in earth, it does. Woman was created from bone [of Adam]; man was created from earth. Hadar Zekenim says that it is because she was used to smoke from her father's house.
Oznayim la'Torah: The angels beseeched HaSh-m to spare Yitzchak so that His promise to Avraham would be fulfilled, so HaSh-m switched the Shechitah for Yisurim (See Oznayim la'Torah - "va'Tich'hena Einav" #2 for more details).
Michtav me'Eliyahu (Vol. 5, p. 434): Why should their tears harm him? Had Yitzchak seen only Ratzon HaSh-m, there would be no reason for tears. He saw Mashehu importance to his body. Therefore, their tears caused him to lose his vision.
That was when he began to go blind, and it was only now that the blindness culminated (Targum Yonasan). See Perush Yonasan.
Why did Yitzchak's eyes dim?
Maharal #1 (Nesivos Olam, Nesiv ha'Tzedek, Ch. 3, p.143): The Gemara (Megilah 28a) derives from here that it is forbidden to gaze at the face of a Rasha; doing so forges a connection with his evil. The eyes dim, because a Rasha ought not to exist, and by gazing he connects with that void.
Maharal #2 (Derech Chayim 5:4, p. 225): Yitzchak was the middle of the three Avos, and his life indicated what would happen in the middle of Jewish history. At the outset he enjoyed prosperity, and at the end he endured suffering; so too, our eyes would be darkened by exile and the Churban.
What was the occasion that prompted Yitzchak to bless Esav now?
Targum Yonasan: It happened to be the fourteenth of Nisan (Erev Pesach), 1 when the angels praise the Master of the World, and the stores of dew are opened - a most auspicious time for Berachos to take effect.
Refer to 27:2:1.
Malbim (to 26:34): Due to Esav's evil wives, Yitzchak understood that he is not worthy of the spiritual Berachos of Avraham. Therefore, he wanted to bless Esav with Berachos of this world. 2
Why did Yitzchak refer to Esav as "his big son"?
Da'as Zekenim #1, Hadar Zekenim #1: This teaches that we may flatter Resha'im in their time [of importance]. 1
Da'as Zekenim #2, Hadar Zekenim #2: It is because he had (was about to be blessed with) the entire world.
Yitzchak said to him only "Beni"! Also Rashi (to Ovadyah 1:2, based on Bereishis Rabah 65:11) comments - 'His father and mother may have called him 'my big son,' but in My eyes he is small!' Perhaps Yitzchak called out 'my big son [come];' and when he came, he said only "Beni." (PF)
Oznayim la'Torah: Yitzchak may not have known that, but HaSh-m spared him from lying, and he addressed him by what he really was without being aware why.
Oznayim la'Torah: As Chazal say regarding Yibum, 'Mitzvah b'Gadol ha'Achim.'
The added phrase, "his eldest son" (Beno ha'Gadol) seems unnecessary.
Maharal (Gevuros Hashem Ch. 8, p. 48): Our Sages point out that only Esav's parents called him 'Gadol;' 1 whereas HaSh-m called him 'Katan... Bazui' (Ovadyah 1:2). Edom would be a despicable nation; they only appear to be great. They have no constructive existence, only falsehood.
Rivkah also refers to Esav as 'ha'Gadol,' in 27:15.
Why does it say he called "Es Esav," and not 'El Esav'?
Ohr ha'Chayim: He could not see him, so he could not call to him. He merely called out his name. 1
We find that he called "El Yaakov" (28:1) afterwards! Perhaps he heard Yaakov learning, or Yaakov had a fixed place to learn, and he knew that he would be there. (PF)
Why did Yitzchak want to bless specifically Esav?
Ohr ha'Chayim: He wanted to bless him because he was the Bechor. He did not know about the sale. He thought that the Berachos will help him to improve. Yaakov was punished for withholding Dinah from him, for she could have improved him. 1 This shows that it was possible!
Malbim: The chosen nation has an elite (Shevet Levi) who are closest to HaSh-m, and the others work to support them. Yitzchak intended that Esav support Yaakov, so he wanted to bless him with income. The spiritual blessing, to be the elite nation, cannot be passed on. It depends only on being worthy for this. A Berachah cannot help. We find that Yaakov did not bless Shevet Levi! 2
I.e. even though he knew that his Shevet will carry the Aron (refer to 50:12:1:1). Moshe blessed Shevet Levi (Devarim 33:8), i.e. after it was chosen. Even though Yaakov put Shimon and Levi together, and it seems that he did not bless them due to the episode in Shechem and the sale of Yosef, presumably, he would have blessed Levi if the spiritual blessing could be passed on. (PF)
QUESTIONS ON RASHI
Rashi writes: "[Yitzchak's eyes] dimmed - due to the smoke of these [wives of Esav, who would serve idols]." Why was only Yitzchak affected? Why specifically in his eyesight?
Gur Aryeh: Yitzchak had intense holiness, having been sanctified as an offering. Eyesight is holy and intangible (i.e. related to the intellect rather than the physical). His eyes could not tolerate being sullied by idolatry.
Rashi writes: "[Yitzchak's eyes] dimmed - due to the smoke of [Esav's wives]." How is this derived from the Pasuk? Perhaps there is no connection?
Gur Aryeh: The verse could have mentioned Yitzchak's blindness at the point that it becomes relevant in the story - "Perhaps father will feel me..." (27:12). It writes it here to tell us the cause of his blindness - Esav's wives (26:34-35). 1
Gur Aryeh adds that according to Rashi's subsequent explanation, Yitzchak's blindness is mentioned here to teach us its purpose - so that Yaakov would receive the blessings.