Dark menstruation that lasts for 2days what does it mean?

Dark blood is: usually older blood being cleaned from the uterus. The main reason for light periods is change in hormone levels. Causes for what seems to be a light period can include: thyroid problems, eating disorders, excessive exercise, initial periods for a girl, use of an intrauterine device, oral contraceptives, perimenopause & implantation bleeding in early pregnancy which rarely occurs.