For bone over a black basecoat I put down a layer or 2 of VMC Cork Brown just to cover the black undercoat. Then over that I use VGC Bonewhite in 1 or 2 thin layers, then I shade with a very watered down wash of smoke paint(20%paint 80% water, with a dab of slow drying medium) then rehighlight with Bonewhite, then layer with bonewhite mixed with white, then just white.
It is hard to get good coverage of a light colour over a dark colour, especially black. It is important to take it in steps from darker shades which cover well, to lighter and lighter shades.