Just like what Dr Ang has mentioned, dark-colored menstrual blood can be considered normal, and you do not have to worry if you do not experience other accompanying symptoms.
However, from a holistic approach, a TCM practitioner would inquire further to find out if you suffer from menstrual pain and if you do, does it happen before, during or after the period; is your menstrual cycle regular; any fertility issues etc.
According to TCM, dark menstrual blood is seen as stagnated blood (血瘀). It may be due to poor circulation (coldness) in the womb where one may be born with a 'cold' body type, or tends to binge on chilled/raw food and drinks.
Typically, a physical consultation is necessary to determine which type you belong to, before a treatment plan can be formulated specially for you.
Do speak to your healthcare provider about your concerns so that it may be addressed effectively.
I hope it helps!
Best wishes,
Physician Lee