Adam Clarke on Hebrews 7:6

Blessed him that had the promises - This is a continuation of the same argument, namely, to show the superiority of Melchisedec; and, in consequence, to prove the superiority of the priesthood of Christ beyond that of Aaron. As in the seed of Abraham all the nations of the earth were to be blessed, Abraham received a sacerdotal blessing from Melchisedec, who was the representative of the Messiah, the promised seed, to show that it was through him, as the high priest of the human race, that this blessing was to be derived on all mankind.