Here's what Tim Baker says about black walnut (which reinforces what barebo said):
WALNUT: black .55. Semi-ring-porous, easy to work, elastic for its mass, similar in performance to cherry, but more tension-safe. Will try to chrysal where cherry won’t. A wonderful, overlooked bowwood. 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% or so of limb thickness, creating appealing contrast with the almost black belly. Very high heartwood extractive level, so as with similar woods, it may be more resistant to water absorption. It’s reported not to warp with rising and falling humidity, possibly for this reason.
There's a fellow by the name of Jimmy Blackmon who has some interesting YouTube videos in regards to archery (some selfbow build alongs, some that talk in depth about gap shooting, etc). Here's a video about a walnut selfbow he made (it's all sapwood in the limbs with some of the heartwood in the stiff handle).
https://youtu.be/luQiJuJ9epI