What To Do For Having Lot Of Mucous At Back Of Tongue?

I contracted a severe cold virus a few months ago, have recovered from it but am left with a build-up of mucous at the very back of my tongue. I brush my teeth regularly including my tongue. My gag reflex kicks in if I try to brush back too far on my tongue. I have tried gargling with salt water, Listerine, and simply drinking more water, I saw my doctor today and she thought it might need to be biopsied. When viewing it with a small flashlight it is dark black. This is the same color of what I scraped off my tongue a few weeks ago. There is no smell from it. I was coughing ALOT and could never seem to clear my throat when I was getting over the cold virus.