Blessed Seed

This is a two-fold promise. The first promise is to all who are descended from Abraham through Isaac,  and Jacob, as confirmed in Acts 3:25.    
The second promise is to the faithful believers not of Israelite lineage, as seen in Galatians 3:8,9. 

The promise of this blessing was made manifest in another descendant of Abraham: Jesus Christ. While both Joseph and Mary were descendants of the house of David, it is through Mary's bloodline that Jesus was born, thus becoming the Son of David and, therefore, able to sit on David's throne. Galatians 4:4.
Furthermore, in Galatians 4:4-7, we who believe that Jesus Christ Is Lord, are now grafted into Him (John 15:4, 5; Romans 11:24) and have become of the seed of Abraham.  
Galatians 4:4-7
But when the fulness of the time was come, God sent forth His Son, made of a woman, made under the law, 
To redeem them that were under the law, that we might receive the adoption of sons.
And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying, Abba, Father. 
Wherefore, you are no more a servant, but a son; and if a son, then an heir of God through Christ.
John 15:4-5
Abide in Me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can you, except you abide in Me. 
I AM The Vine, you are the branches: He that abides in Me, and I in him, the same brings forth much fruit: for without Me, you can do nothing. 
Romans 11:24
For if you were cut off of the olive tree which is wild by nature, and were grafted contrary to nature into a good olive tree: how much more shall these, which be the natural branches be grafted into their own olive tree?
Ephesians 2:11-13
Therefore remember that formerly you—the Gentiles in the flesh, the ones being called the uncircumcision by that being called the circumcision, made by hands in the flesh--
that at that time you were separate from Christ, alienated from the commonwealth of Israel, and strangers to the covenants of the promise, not having hope and without God in the world.
But now in Christ Jesus you, the ones once being far off, have become near by the blood of Christ.

We are blessed through our faith in Jesus Christ. And as long as our faith remains in Christ, and we continue in obedience to God's Word, we will be forever receiving blessings of promise from God.