The clause entreat the favor of God
could be interpreted to mean beg the forgiveness of God.
If this is the meaning, then the prophet is calling the priests to repent of their sin of offering blemished sacrifices. But it is more likely that the prophet is speaking ironically here in the sense of, Go ahead! Try asking God to look favourably upon you while offering such sacrifices! Do you think God will look favourably upon you if you offer him blemished sacrifices?
The Hebrew phrase used here is used elsewhere in the sense of seeking the Lord (Zechariah 8:21–22), which refers to worship in general (1 Chronicles 16:10–11; 1 Chronicles 22:19; Psalm 34:10). The point that the prophet is making is that such compromised worship of God is unacceptable to God, or worse! It is evil and abhorrent to God! God will by no means show favour or grant his blessing to those who offer compromised worship. The opposite is implied. God will look upon them with disfavour, in wrath!
9 And now entreat the favor of God, that he may be gracious to us. With such a gift from your hand, will he show favor to any of you? says the LORD of hosts.