They. That is, the 24 elders and perhaps also the 4 beasts (see below on “us”).
A new song. The song was new in the sense that it was entirely different from any sung before. This expression is common in the OT (see Ps. 33:3; 40:3; Isa. 42:10). Here it is particularly fitting, for it represents the song growing out of a unique experience— salvation through the victory of Jesus Christ (see on Rev. 5:5). It is the “new song” of those who have a “new name” (chs. 2:17; 3:12), who inhabit the “new Jerusalem” (ch. 21:2), when all things are made “new” (ch. 21:5).
Worthy. See on v. 2. The heavenly chorus leads out in the recognition that God has been vindicated from the accusations of Satan, through the victory of His Son. Some see in the 24 elders representatives of the saints, who themselves were once captives of evil. The saints appear before the onlooking universe as witnesses to the righteousness and the grace of God. See on Rev. 5:5; cf. Eph. 3:10.
Thou wast slain. The death of Christ, with its resulting salvation for man—which in turn vindicates the character of God—is the basis for the worthiness of Christ (see on v. 2).
Us. In spite of the fact that textual evidence attests retaining this word, translators and commentators generally omit it on the basis of other textual evidence in v. 10 (see comment there).
By thy blood. See on Rom. 3:25; 5:9.