Albert Barnes on John 20:29

Because thou hast seen me. Because you have looked upon my body, and seen the proofs that I am the same Saviour that was crucified. Jesus here approves the faith of Thomas, but more highly commends the faith of those who should believe without having seen.

Blessed. Happy, or worthy of the divine approbation. The word has here the force of the comparative degree, signifying that they would be in some respects more blessed than Thomas. They would evince higher faith.

That have not seen, &c. Those who should be convinced by the testimony of the apostles, and by the influences of the Spirit. They would evince stronger faith. All faith is of things not seen; and God blesses those most who most implicitly rely on his word.

(g) "blessed are they" 1Pet 1:8