At that time; the time when the covenant was restored at Sinai, not the time when Aaron died. The appointment of the tribe of Levi for service took place in connection with that of Aaron and his sons to the priesthood (Numbers 3:4).
The service to which the tribe of Levi was chosen appertained to the tribe as such, including the priests as well as the non-priestly Levites, though parts of it specially belonged to the one class rather than the other.
Thus the bearing of the ark was the special duty of non-priestly Levites, the Kohathites (Numbers 4:4, etc.; 1 Chronicles 15:15); but was also, on peculiarly solemn occasions, discharged by the priests (Joshua 3:6, etc.
; Joshua 6:6; Joshua 8:33; I Kings Joshua 8:3, Joshua 8:6, etc.). To stand before the Lord to minister unto him was the special function of the priests; but as the service of the Levites was also a sacred service, they too are said to stand to minister before the Lord (Deuteronomy 18:7; 1 Chronicles 15:2; 2 Chronicles 23:6; 2 Chronicles 29:4, 2 Chronicles 29:5, 2 Chronicles 29:11, 2 Chronicles 29:12).
To bless in his name does not mean, as some propose, to invoke the Name of God, or to praise his Name, but to pronounce a benediction or invoke a blessing on the people in his Name (cf. 2 Samuel 6:18; 1 Chronicles 16:2).
This was the special duty of the priests (cf. Numbers 6:22-27; Deuteronomy 21:5; 1 Chronicles 23:13), but might also be done by others (as by David), and in this benediction the Levites might join (2 Chronicles 25:27).