In what way will "the nations be blessed" in Galatians 3:8 and when?

That verse, and the one following it, answer your question. Here's the New Living Translation rendition:

"What's more, the Scriptures looked forward to this time when God would declare the Gentiles to be righteous because of their faith. God proclaimed this good news to Abraham long ago when he said, 'All nations will be blessed through you.' So all who put their faith in Christ share the same blessing Abraham received because of his faith."

The cross reference to the ancient promise to Abraham is in Genesis 12:3

The NLT footnotes add; "By faith, Abraham foresaw the coming Messiah (cp. John 8:56) and received the message that 'All nations will be blessed through you' (Genesis 18:18 & 22:18). trough Abraham's offspring, the Messiah, God would accept all nations on the same basis as Abraham himself - by faith."

(PS You cannot give extra points. You can only give 10 points if you choose a best answer, but I've never yet seen anyone promising to give BA actually do it. They always let the question go into voting.)

Of vital importance to all mankind is the blessing concerning Abraham and his Seed. (Ge 12:3; 18:18; 22:18) Jehovah blessed Abraham and Sarah by miraculously renewing their reproductive powers, enabling them to have a son in their old age. (Ge 17:16; 21:2) He prospered Abraham and used him in a pictorial way to foreshadow greater things. (Ga 4:21-26) Therefore, God’s blessing in providing a seed for Abraham has higher significance in the promise that the people of all nations will be blessed by means of the one Isaac foreshadowed, Jesus Christ.—Ga 3:8, 14; Ac 3:25, 26; Heb 6:13-20.

http://wol.jw.org/en/wol/d/r1/lp-e/1200000772?q=Ga...

The New Covenant

12 Jehovah foretold a new arrangement for his people when he said: “This is the covenant that I shall conclude with the house of Israel after those days. . . . I will put my law within them, and in their heart I shall write it. And I will become their God, and they themselves will become my people.” (Jer. 31:31-33) By means of this new covenant, Jehovah’s promise to Abraham was to have a glorious and lasting fulfillment.—Read Genesis 22:18.

13 Jesus referred to this new covenant on the night before his death when he said: “This cup means the new covenant by virtue of my blood, which is to be poured out in your behalf.” (Luke 22:20; 1 Cor. 11:25) Are all Christians included in this new covenant? No. Some, like the apostles who drank of that cup that evening, are participants in the new covenant.* Jesus made another covenant with them to rule together with him in his Kingdom. (Luke 22:28-30) They will share with Jesus in his Kingdom.—Luke 22:15, 16.

14 What of those who will live on earth under his Kingdom? They are beneficiaries of the new covenant. (Gal. 3:8, 9) Although not participants, they ‘lay hold’ of this covenant by submitting to its requirements, just as was foretold by the prophet Isaiah: “The foreigners that have joined themselves to Jehovah to minister to him and to love the name of Jehovah, in order to become servants to him, all those keeping the sabbath in order not to profane it and laying hold of my covenant, I will also bring them to my holy mountain and make them rejoice inside my house of prayer.” Jehovah then says: “For my own house will be called even a house of prayer for all the peoples.”—Isa. 56:6, 7.

http://wol.jw.org/en/wol/d/r1/lp-e/2010202?q=Galat...