No, Donatists denied that a person can repent out of apostasy or that sinful priests can have grace. Still Saul lost the grace of Holy Spirit at some point and so did Judas. He did not carry the power of Apostleship into the Hell.
You cannot have Black Mass since God will not mix with evil.
As long as there is valid matter (i.e. valid bread and wine), and valid form (i.e. "This is my body"), and the public intent is to perform the Eucharist, then there is a valid consecration.
If the priest is an apostate and enemy of God in secret, the miracle of consecration can take place on behalf of faith of the those present. But if the apostasy is open the believers may not follow it. God cannot be mocked.