Although wither skeletons have black bones they still drop white bones.
Possible fix: A second "datavalue" for a bone that has a black texture and can't be crafted into "normal" white bonemeal but black bonemeal that grows netherwart.
Comments 3
1. This is a feature request. This place is for bugs only. Bugs, exploits, and bugs. No feature requests! 😞
2. Please don't mark things as affecting future versions; you couldn't have possibly tested them.
This is not a bug, but a feature request, and should be posted on the minecraft forums