If I left the sapwood on, I wouldn't get the heart wood and the awesome transition appearance.
Plus the TBB 4 (P.54), says that " Bows can be all sapwood, or all heartwood, or mixed, sapwood taking a bit more set in compression. The off white sapwood can be worked down to 25% of limb thckness creating apealing contrast with the almost black belly"
Also "If a thin layer of off white sapwood is left on the back, such bows are not just beautifull, but strikingly beautiful"
So, its been done, and it seems as though that's the recomended approach with BW.
I have a sister stave that I'll probably make from just sapwood but I wanted to try this out and see how it works. I'm thinking about staining the back so the grain comes out so I can see the rings better IDK.
If I just can't get a single layer of white sapwood, I'll just work down all the way to the heartwood, which is easily seen in stark contrasts to the heart wood.