Usually with black locust (and osage and mulberry) you take off the sapwood and use the heartwood, but this one is so narrow that you have to leave the sapwood. That means, as already said above, just take the bark off and that is your back (which is how you treat whitewoods like elm, hickory, maple, etc.). I have had issues with the sapwood checking, so I would probably leave the bark on for now. It is harder to remove later, but not a big deal. If you take if off now, definitely seal the back well.