I think you can indeed use either sapwood or heartwood for the back, but with black walnut, leave yourself more width to start with than you think you'll need. I haven't worked it much, so don't take this as gospel, but I would try to get heartwood on the belly. It's fine wood, but I had both of the two bows I've made from it just act kind of..... softer than it is. Didn't take huge set, but it was like the wood just wasn't wanting to be stiff.
