Albert Barnes on Hebrews 9:20

Saying, This is the blood of the testament. Of the covenant. Heb 9:16,17. That is, this is the blood by which the covenant is ratified. It was the means used to confirm it; the sacred and solemn form by which it was made sure. When this was done, the covenant between God and the people was confirmed --as a covenant between man and man is when it is sealed.

Which God hath enjoined unto you. In Ex 24:8, "which God hath made with you. The language used by Paul, "which God hath enjoined" -- ενετειλατο-- commanded--shows that he did not regard this as strictly of the nature of a covenant, or compact. When a compact is made between parties, one does not enjoin or command the other, but it is a mutual agreement. In the transactions between God and man, though called Berith--or διαθηκη--diatheke--the idea of a covenant or compact is so far excluded that God never loses his right to command or enjoin. It is not a transaction between equals, or an agreement; it is a solemn arrangement on the part of God which he proposes to men, and which he enjoins them to embrace; which they are not indeed at liberty to disregard, but which, when embraced, is appropriately ratified by some solemn act on their part. Compare Heb 8:6.

(a) "blood" Mt 26:28