Sforno on Genesis 27:2/ספורנו על בראשית כ״ז:ב׳

1לא ידעתי יום מותי. Although the blessing will prove more effective the closer the one dispensing is to death, as was the case in the blessings dispensed by Yaakov and Moses, seeing that their souls were already closer to heaven at the time when they dispensed the blessing, Yitzchok was not aware of when he would die so that he could to time his blessing accordingly.