Dark, long tastebuds. Hairy tongue happens when some of the taste buds on the top of the tongue get very long and discolored. The discolorations can come from food, tobacco and/or bacteria. Using a tongue cleaner over the affected area several times a day for several days can sometimes help.
Google it. Should be able to find a photo of it. Basically looks like brown or black hairl growing in the center of the tongue. Start stroking your tongue with a toothbrush and a lot of tooth paste.