Difficult. Has to dry completely and then sand to expose bare wood. To get rid of the black, either oxalic acid gel or dilute hydrogen peroxide. You need to watch it doesn't go to far before neutralising it and you may need to re-stain the area to get to a decent colour match. Any rot, cut out and replace the board. Then refinish the area with whatever is on the rest of the floor, or sand the whole floor and redo the finish from scratch.
Basically it's very tricky to get an invisible fix. There are specialists in wooden floor refinishing out there who do this - experience helps a lot.
