What Causes Black Discharge During Periods?

I am a 21 year old female who had a child 7 months ago, and am still breastfeeding. About two months ago I started my period, and while it has been regular in terms of days it's supposed to be apart, it has lasted five days, as opposed to my usual seven, and both times it has been black with a consistency similar to tar, with no hint of red. Is this something I should be concerned about, and if not, how long should it continue before I should be concerned?