Sometimes during preriod, the blood will clump and almost look like a cloth. This is normal so you will see this and it is so dark will appear like black spots. If you have heavy bleeding and period is lasting too long, this is when you need to get checked out. Otherwise, I would not be too concerned
You don't say when in the cycle, but they are probably 'old' blood. Tiny little particles of blood. If you are really worried, see your doctor who will explain.
They are blood clots, very normal material. They could also just be dried blood, like karmabum said.