What does very dark red blood in menstrual cycle mean?

Color of blood: The color of the blood from a menstrual cycle has to do with its components. A cycle is the lining of the uterus sloughing off because it is not being used to house an embryo (combination of an egg and sperm). Every month your body prepares itself to be pregnant and when it is not the uterus expels the unused material, which is your period.

Normal: Menstrual blood is not pure blood. It is both arterial and venous and it is also mixed in with the sloughing endometrium (the inner uterine lining). That's why it has the appearance you describe.

Older blood: When the menstrual period occurs, the blood may be slow in flowing out. The blood will tend to be darker as it gets older, while fresh blood will be bright red. What you describe sounds normal.