Yaakov's blessing arranged by Hashem

מה שהי' צריך להיות ברכת יעקב שלא ברצון יצחק.

The reason Yaakov's blessing had to come about against Yitzchak's conscious wish.

The Sefas Emes asks why it had to be that Yaakov received the blessings in a way Yitzchak did not intend — by appearing to him as Eisav. There must be a deeper meaning in this.

נראה ע"י שבברכה זו תלוין כל העולמות ולא שייך שיהי' ע"י איש רק ע"י הקב"ה שאין זה דבר שבמקרה.

It seems that, since all the worlds depend upon this blessing, it could not properly come about through any human being but only through HaKadosh Baruch Hu — for this is no matter of chance.

The brachos given to Yaakov carry the whole of creation. Something of such cosmic weight cannot rest on a human decision; it had to be arranged directly by Hashem so that it would not appear to depend on Yitzchak's will at all.

[ואפשר עי"ז הי' הברכה ע"י יצחק שכהו עיניו כמ"ש רש"י ז"ל שייחד הקב"ה שמו עליו כנ"ל]:

[And perhaps it was for this very reason that the blessing came through Yitzchak whose eyes had grown dim, as Rashi z"l writes — that HaKadosh Baruch Hu rested His Name upon him, as above.]

Yitzchak's blindness was itself part of the plan: precisely because he could not see, the blessing was channeled by Hashem rather than by human sight, so that Hashem's own Name and providence rested on the event.

Summary: Because Yaakov's blessing upholds all the worlds, it could not depend on human choice; Hashem Himself arranged it — even using Yitzchak's blindness — so that the bracha would flow directly from Him and not by chance.