It seems Warlocks can breather easy! Last week it seemed Blizzard nerfed the Hellcaller Hero talent tree, putting a 5 second cooldown on 
Blackened Soul. The result was unintended and wasn’t supposed to affect gameplay at all. It is now being reverted.
Hello!
This was an unintended change and a fix for it has been deployed.
The original change was intended to prevent Vile Taint from activating Blackened Soul for every enemy it affects by providing a small enough internal cooldown that would have no effect on gameplay.
Thank you for bringing this to our attention.