I smashed 7 fingers a week ago. The nails are half black-blue from the bottom up. Is it too late to prick them or has the blood dried up?

Leave. Them alone. Should resolve in time .Be more careful next time. Go see your doctor, if new signs or symptoms arise.
Likely clotted. The blood is likely clotted by now and will not drain by pricking. Pricking as self treatment is not recommended anyway as it is likely to introduce infection.