Is it spotting or heavier bleeding? Spotting can be normal, every woman will be different- but sounds like you might want to go get checked out just to make sure your hormone levels are in check.
Fibroids or cysts but see a doctor. Once you have this black blood its hormonal. I have the same problem caused by the fibroids, yours maybe caused by polyp. It is stale blood but there is a reason for it. All the best.