Your interpretation of the ending sort of depends on who you are as a reader. It seems, on the whole, to be pretty negative: Black never receives the inspiration he needs from God—he can't find the words that will reach White's heart—so White walks off, intending to commit suicide.
We can take Black's promise to "be there in the morning" as a feeble gesture, but maybe he will find a way to reach White—it's not entirely impossible that God will answer Black's question at the end and send him off with a decent argument that will finally reach White, improbable as that may seem. White's not dead yet, after all. The most important part of the ending, though, is probably that Black doesn't lose faith. He's been crushed by his defeat, yet he doesn't give up hope. He's still ready to do what he has to do… but then again, so is White.
Looked at this way, the play ends on a note that both honors faith and calls faith into question. We see it acting as Black's support in a hard time as much as we see it driving White to commit suicide.