What you have is usually referred to as Black Hairy Tongue. It is a benign condition and the cause is not known. The cells of the top layer of the tongue get enlarged, then don't normally slough off, resulting in kind of a thick mat. This extra tissue can get stained by food materials, bacteria, or tobacco and become yellowish, brown or black. You just need to take a little extra care to brush your tongue or rinse your mouth. Sometimes Black Hairy Tongue will disappear spontaneously.