If by Black Magic you mean Shadow Magic then never... I seriously doubt Kingsisle will ever make Shadow Dispels since: 1) It's unneeded. 2) Shadow Dispels would mostly be for PvP 3) Shadow Magic has many counters. Examples: Pre-Cast: Black Mantles. Post-Cast: Dispels, Mantles, Smoke Screens, Stuns, Weakness, Etc. So Really Shadow Magic doesn't need any dispels and I hope that Kingsisle will never make a dispel for Shadow Megan Frostflame Level 100 Child of Light and Shadow
When do we get Black Magic Dispels in the game.... ?
So far in the game we have nothing titled "dark magic", but I do believe you are talking about Shadow Magic. As far as I know, there is no dispels of that. Death Magic dispel is taught by a secret trainer on Colossus Boulevard. Hope this helped!