To me, Cullen's poem shows an acceptance of fate, but one tinged with regret. I don't really see anger or celebration but more or resignation.
To understand why, look at all the things he compares to the condition of a black poet. He compares it to be being a blind mole, to being Tantalus or Sisyphus, both of whom are being tortured for eternity. Give this, I don't really see how this can be triumphant.
So, to me, Cullen is saying that he doesn't understand why God would make him this way, but the last quatrain and the couplet imply a bit of bewilderment and acceptance more than anger.