I bring up grey/black phloem and I don't smoke. Doctors aren't helping. Any ideas?

I've been to the doctors about this several time.. and every time they just seem to be like 'hmm.. how weird' and then don't really believe me. But it's not "normal" to be bringing up phloem ten times a day with black dots or black sections in it or it's grey or both! I don't know why and it's not causing me any pain or discomfort, so it's not like if I don't find out then I'll die or something, but it's not normal surely?! Is there some sort of expert I can go to about it?