Did you mean death dispels (aka Strangle)? You can train that and all the other dispels on Colossus Blvd from the "secret" vendor. But that's really not a question for this forum, this is for ideas.
If you meant to suggest that there should be new dispels, like for shadow magic, then you're in the right place.
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!