I woke up and over night the bottom of about 6-7 teeth had turned black. is there a reason for this? i brush 3-4 times a day. look at my teeth daily.

Black teeth: Their is no way to answer this without a clinical and visual exam. It could be stain, decay, tarter buildup, infection, etc. Call your dentist for an examination.

Overnight?: What you describe seems unusual. Perhaps what you see was already present and you just noticed it. There are many possibilities and this would have to be determined by a personal one on one clinical exam by your own dentist. Make an appointment and find out.