I had the same issue a few years ago, so I asked my doctor. He told me that dark red/blackish blood is quite normal; it just means that the blood/tissue has been exposed to the air and has oxidised more quickly.
Nowadays, my periods/withdrawal bleeds are very short; only about two days, and a lot of the time it's brownish/black. If it really does worry you, then I would absolutely talk to your doctor, but for the most part, it's normal. The blood has either oxidised quickly, or there's always the chance that it might be OLD blood that didn't make it out of the uterus during your last withdrawal.
Hope this helps and makes you feel relieved!