After you apply the bluing or blacking, you could try coating the area with some lacquer. That will help it last longer, I think.
Slightly related note: I understand that it looks funny, and hey, this is your sword, you should be happy with it, but I wonder if it's strictly wrong? I mean, if you rubbed the surface of old tarnished silverwork enough, would it get bright(ish)? Or do you need polish for that? I'm not telling anyone what to do with their sword, I'm just wondering for my own knowledge.