I Hope So. For 3-4 Weeks, I Develop A Black

I hope so. For 3-4 weeks, I develop a black coating on my tongue. At first, I thought it was because I didn't brush my teeth at night and drink grapefruit juice before bed. However, it reappeared after brushing, scraping my tongue and only drinking water. I brush at least 3 times a day, rinse with peroxide and mouthwash. I sometimes brush more often. any suggestions?