The colour of your period blood can vary from cycle to cycle. As a rule of thumb, the color of menstrual blood is a shade or two darker than "normal" bleeding, like say, if you were to cut yourself.
As Dr Paul has mentioned, the color of your menstrual blood is essentially a sign of how quickly the blood is passing through.
The brighter red it is, the more recent the bleeding and the quicker the blood is passing through your cervix and into the vagina.
The darker the flow (which can range from dark red to brown) the more likely it is to be older blood, or experiencing slower flow.
This is because when blood takes extra time to exit the uterus, the iron inside blood oxidizes, which causes it to appear dark brown or black in color. It may even resemble coffee grounds.
Very occasionally, dark coloured period blood could be a sign of other things, such as an infection. This is usually accompanied by foul-smelling discharge. Thus, if you are concerned, you should probably see a GP and explain your symptoms, as well as your history.