well, you could try to lay down a sheet of paper on either side and carefully spray in some black, or use a bendable ruler to allow you to do a black magic marker line... and I think if you first hit it with some acrylic clear followed by lacquer you'd probably be ok... or you could just replace that section with a new piece and try to blend the joins with acetone/melted-binding... but in all honestly I think this would be a complete 'redo' for me. Binding isn't ever really 'fun' but it's not all that hard. It looks like that is on the top side... and I would look at it every time I play the guitar... and it would get under my skin.