The darkness is likely from the oxidation on the edge of the maple from the steaming process. You won’t be able to sand out a glued edge on this type of joint. You’ll just be chasing this darker spot all the way until there is no joint left. The best solution is to tidy up the joint as evenly as possible. You could stain the rim darker to help draw less attention to the joint. Any steam bent rim is going to have a glued joint somewhere.
I almost always set the tailpiece over this joint so it’s less noticeable.