Anything that has trouble taking the glue should be "sized" by wiping thinned glue over it before the actual glueing process. In some glues that will require glue up while still damp.
On a belly, you can burn the wood black without damaging it deeper by torching it and not giving it time to soak the heat in. Surface charring that wipes off with sanpaper, basically. Toasty bown wood will not present you any problems, even if you are glueing to the toasy part. I know this bwecause I ran several elm and hickory slats though my friend's commercial pizza oven for tempering, and had fine results, except that unrestrained slats of wood milled down from saplings, going through a pizza oven, warp like a mother-hen. That, and very dry wood does not like to bend into recurves once toasted.
The right color is toasty, caramel brown. I have had selfbows survive despite some very dark spots, even basically black, on the belly. But I don't recommend it.
