Black flame is generally used to deal very rapidly with foes or also to make the final hit on bosses or minibosses. It has not to be understimated because it's one of the most reliable Pyromancies in the game.
Although relatively short on range, the black flame offers a decent amount of Dark damage when dealt to an opponent. The player must consider the casting time, as the interval leaves the opponent susceptible for attack: use it wisely!
Using Black Flame with 40 in both Intelligence and Faith and at least the Witch's Ring, alongside the Pyromancer's Parting Flame, will result in massive damage, every cast doing around 500 damage to the opponent, thanks to the 'flame burst' that the Pyromancer's Parting Flame produces when used to cast some specific Pyromancies.