Thanks for the query.
Sometimes urine pregnancy
test may not detect early pregnancy.
So you once go for ultrasound
to find out the possibility of pregnancy.
If there is no pregnancy, then the bleeding you are having could be scanty flow during periods.
That can occur due to many causes like anemia
abnormalities, PCOS etc.
Black color can be seen due to collected blood in the uterus.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429