What is the ruling on initiating marital intimacy while doubting the presence of menstruation, followed by penetration and the sight of blood?

There is no sin upon you both for what occurred, as long as you did not intentionally engage in intercourse while knowing of the menstruation, as indicated by the verses. You both stopped intercourse after becoming aware of it, and there is no consideration for the doubt that occurred before it, for the original state is the absence of menstruation.