Dark Petition adds colorless rather than black mana to your mana pool. My converge spells, for example, won’t gain colored mana.
Yes, I know it works like the card says it does. And yes, I know it’s a really little thing. But black cards gaining black mana is much more thematic to MTG, especially now so much of the design space of colorless mana in MPQ has been colonised by Eldrazi.