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

Question

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.

Answer 1

I think what you say is mostly true... until the avos:

"וְאֶעֶשְׂךָ, לְגוֹי גָּדוֹל, וַאֲבָרֶכְךָ, וַאֲגַדְּלָה שְׁמֶךָ; וֶהְיֵה, בְּרָכָה."

והיה ברכה - הברכות נתונות בידך, עד עכשיו היו בידי, ברכתי את אדם ואת נח ואותך, ומעכשיו אתה תברך את אשר תחפוץ.

(Rashi, beginning of Lech L'cha)

God specifically gives over the power of blessing to the Avos, which is why they and their descendants have that ability. Though it seems it existed in a more limited fashion before also, e.g. Noach did bless and curse his children.

Answer 2

There is a difference between a blessing from God and a blessing from another person. Consider the blessing given by the kohanim:

כֹּה תְבָרֲכוּ אֶת בְּנֵי יִשְׂרָאֵל אָמוֹר לָהֶם. יְבָרֶכְךָ יְהוָה וְיִשְׁמְרֶךָ. יָאֵר יְהוָה פָּנָיו אֵלֶיךָ וִיחֻנֶּךָּ. יִשָּׂא יְהוָה פָּנָיו אֵלֶיךָ וְיָשֵׂם לְךָ שָׁלוֹם. וְשָׂמוּ אֶת שְׁמִי עַל בְּנֵי יִשְׂרָאֵל וַאֲנִי אֲבָרֲכֵם

This is how you shall bless the children of Israel, saying to them: "May the Lord bless you and watch over you. May the Lord cause His countenance to shine to you and favor you. May the Lord raise His countenance toward you and grant you peace." They shall bestow My Name upon the children of Israel, so that I will bless them.

Why are they blessing the people that God should bless them? Why don't the kohanim bless the people themselves?

The answer is that they are. A blessing from God is His granting you with good things, His "guarding you and favoring you". A blessing from a Person A to Person B, on the other hand, is Person A's prayer to God that He should bless Person B. This is the case with the blessing of the kohanim, and it is the case with any human blessing another.

Answer 3

I once heard this as a natural outcome of some of the Midah K'neged Midah (A ways of saying that punishments fit the crime) systems that work in the world. When you speak Lashon Harah about another person, you bring upon judgement about your self. When you judge people harshly, you yourself get judged harshly. When you bless people, you yourself get blessed. Part of the blessing you receive is that your words are trusted and become true. Similarly, but slightly different, is the idea that "A good name is more precious than jewels." If someone curses you, it is a sign that you have some flaw that needs to be repaired, when someone blesses you, they see you as a person worthy of praise. Again, this comes as a natural outcome to the concepts of 'having a good name' or the inverse problems with the evil eye.

These systems, along with the ability to do Teshuva at any point in our lives are some of the greatest Gifts that Gd bestows upon people.