Or HaChaim on Genesis 48:8/אור החיים על בראשית מ״ח:ח׳

1 וירא ישראל את בני יוסף, Israel saw the sons of Joseph, etc. It is difficult to understand how Jacob failed to recognise grand-sons whom he had tutored for the last 17 years on a regular and frequent basis. Tanchuma on our portion explains that Jacob saw with his mental eye that in the future some evil people would come out of Menashe. This is homiletics, of course. We need to be able to explain the plain meaning of the verse also, and this is why the Torah reveals here that Jacob's eyesight had begun to fail him so that he could not see clearly and recognise people. Hence, although Jacob was aware that in addition to Joseph with whom he was speaking two other human beings were standing in front of him, he had recognised Joseph because he had been told in verse 2 that Joseph had come to visit him. He did not recognise Joseph's sons, however, and this is why he asked who they were.

2It is also possible that Jacob wanted to arouse feelings of love in the heart of the children's father before he commenced to bless them. In such circumstances the effectiveness of the blessing is enhanced. By asking מי אלה, Jacob was sure he would elicit the response of a proud father, someone who loved his children.