Hi, Cindy. In answer to your question, darned if I know. Some woods will naturally darken over time, like cherry. Is it possible that this is what happened to the dark area while the suns bleaching action kept the rest light?
There are 2 things for you and your floor expert to consider. You might try the wood bleach in a small area. If is doesn't work, you have not lost much. Did you consider staining with a color match to the dark area? If it is just naturally darkened wood, you should be able to blend the colors enough so that it was not noticeable.