Wood that has water sitting on for an extended period or repeatedly will often turn black. A professional way to take the black out is to use denatured alcohol applied with a piece of 0000 steel wool. (Thanks to Bob Arth, furniture repairer, 763-571-5044, for this tip.) The alcohol will open the pores of the wood so that it can dry out. The steel wool is better than a cloth because it won’t absorb the alcohol. 0000 grain is generally too fine to scratch the wood. But certainly test in a small inconspicuous area before doing a whole project.