Dark blood can be the result of dead cells in the area of bleeding. It may be because there was a clot that did not pass and it chose to pass at this time making if old. Doctors will ask if blood is black when a patient come in from abnormal bleed whether it be vaginally, rectally, or from the nose. This gives them a lot of information toward a diagnosis. It can be the results of internal bleeding and can be most serious. In your case, however, it is likely you have nothing to worry about. When you should be concerned is if you are passing an enormous amount, of any color blood, if you experiences pain, dizziness, reduced appetite, a strong unusual odor or decrease in energy. Don't let this difference control your life too much at this point. Women can experience all kinds of off the wall things at any point in life. best wishes
If you are getting periods regularly and not heavy and on time regardless of its color, then it is normal. The dark color of blood can be due to use o some medicines or deficiency of water. Increase your liquid up take and relax.