I feel like for healing to work there has to be some "life" element to the color, and neither blue nor black have it.  Black moves lifeforce around, and does some healing that way.  

I think a "healer" whose art revolves around stealing from an enemy to heal a friend is a reasonable character, though.  

EDIT: [@RedDusk] Blue brings the physical element to the healing.  Anatomy/physiology knowledge and all that.  Like the stitchers in Innistrad.