They nerfed the raw damage, but upped the scaling:
Blood Plague damage and AP scaling increased by 30%.
Frost Fever AP scaling increased by 30%.
I'm not a theorycrafter, but i'dd guess that's en overall buff, despite the nerf to some of the buffvalues. Specially for unholy.
Bit puzzled about the nerf to scourge strike though. It just seems so minor, that i don't get why they bother.