I found this..When the products of menstruation aren't completely cleared during a menstrual cycle, this results in blackish brown periods/discharge.This particular color indicates "old blood".It doesn't seem to be due to an infection.Another reason you could have a lot of iron in your blood which would make it rich and dark. Most common cause is that it is old blood.The viscosity or thickness increases because of clots in between the blood.As you are having regular periods,there is nothing to worry.If you have irregularity in periods,only then there is need to consult your doctor.