Having said that Ya'akov's eyes were weak from old age, why does the Torah see fit to add that he could not see?
Seforno: Because he wanted to bless Efrayim and Menasheh, and in order to bless with a full heart, one needs to see the object of one's blessing. 1
As we find in Balak (23:13) and in V'Zos ha'Berachah (34:1). (The former was in order to curse! It is clear to Seforno that the same applies to Brachos and curses - PF.)
What is the significance of the fact that Ya'akov kissed Efrayim and Menasheh and hugged them?
Seforno: So that his soul should cleave to theirs, for the Brachah to take effect. 1
Hadar Zekenim: Shechinah dwells only amidst Simchah.
Pasuk 8 said that Yakov saw them! Why does it say now that he could not see?
Moshav Zekenim, Riva citing R. Tam: He saw the form of people, but did not know who they are.
Bechor Shor: He could not understand what he saw.