I've seen that coloration in spring wood before just like you describe and pics you shared. Always in standing dead locust or logs that have been cut and left to weather. In my experiences its in the (spring)heartwood rings close to the sap wood. I have always removed wood to get below that and have had success. I think that black is mold just like you would see in sapwood from being damp. I think you have a good chance of the late wood below being good. Im a sucker for pretty wood but think it to be a too risky to leave that spring wood on there as a back. Your locust looks different than what I seem to find, what Pat B said in his post about doing a test piece +1
only thing I could add that has not been said....if this stave still has moisture to lose, seal the back when you get to it, with shellac. So you don't get drying checks on the back .
Mike