3 of 3 added Rashi

How can people effectively bless when blessing is God's domain?

How do people have the power to bless? Yitzchak's blessings for his sons pretty much came true, and similarly later when Yaakov blesses his children and earlier (in the inverse) Noach curses Canaan. How do mere humans, even if they are the patriarchs, have the ability to make these (correct) pronouncements about the futures of their children? Only God has the power to determine such things. Are the blessers acting on their own initiative or is God speaking through them and they're just the vehicle?

One source for blessing being God's domain is in the Rashi on Gen 35:11:

I am the Almighty God: Heb. שַׁדַּי. For I have the power (כְּדַי) to bless, because the blessings are Mine.

I am not talking about blessings that are actually requests of God, like birkat kohanim. Rather I am talking about the places where a person says "such-and-such will happen to you" and it does.