I'd hold on the whole "END OF DAYS!" aspect of Tranquil Heart until we see what it does. For all we know, we'll only get level 1. That might be -5 Enmity for cures. Okay, so you change your gear some so you'll have more +Enmity on. Done. Now, the Black Magic aspect was pretty stupid, especially if you consider people /RDM or /DRK getting full enmity for debuffs they very rarely landed, if ever. Ideally they could've made it so you only got full enmity if it landed, but then that's also a bit of a tank nerf since you'd have to wait until stuff wore off to try again.