Before I would spend the next few months stripping/ sanding/ refinishing all the woodwork in the whole house (so it all matches)just for a few spots, I would try "Orange Glo". It sounds to me that these blackened areas could just be accumulated oils and dirt from hand contact. I too have a house full of ornate colonial trim, all original and has never been refinished and find the black areas on the bannisters and some door casings (where my son finds it necessary to put his hand every time he goes through the doorway). Murphy's oil soap doesn't touch this stuff but the Orange Glo takes it right off.