To the best of my knowledge there is a principle that if a woman knows with reasonable certainty ( or even reasonable suspicion ) that an emission of blood is from a wound and not from the uterus then she is not rendered niddah. This comes up often for instance in the case of gynaecological exams that can cause non-uterine bleeding.
If this is the case, then why would a woman be rendered niddah ( or safeq zavah or whatever you want to call it ) after defloration considering that we know with certainty that the blood is from the broken hymen.