As far as I know, it is normal. I have had that black blood for the last five periods now, and I am married and inactive.
Some sites say its dry blood, others say it just clotted (got aged) and then came out a bit late, and others say that it is probably best to ask your doctor. But in my personal opinion, if you have no weight changes, or sudden medical issues like black outs, fainting dizziness, nausea etc... then you should be okay. Wait for one more month and see if the black blood comes again. If so, and you yourself personally feel worried about it (which i am not worried about mine, but this is YOU), then go to a doctor... and hopefully DURING the black blood period because they might require a sample.
Keep in mind that I am not a medical professional, but I base this on my own personal feelings. As an added note, i did have a child, and i never experienced said black blood.
Hopefully this helps.