A friend told me today that when people used to say "virgin blood" as a ritualistic ingredient, they actually meant blood that hadn't been used for a ritual before, rather than someone who hadn't had sex before. Is this true?

Linguist here. John Ford's 1633 drama Tis Pity She's a Whore uses virgin blood of "deflowering."

However, blood is often used within the play, not only in religious critiques—such as that of the Eucharist or the virgin blood of deflowering—but also in critiques of Renaissance society as a physical bearer of human characteristics and statuses. (emphasis mine)

Aeschylus also uses "virgin blood," but here parthenou probably means "of a young woman" and not "of a virgin."

Virgil also used "virgineum" to mean "virgin": Hasta ... virgineum bibit cruorem "The spear drew the virgin's blood." Book XI:803-4

I realize that three data points do not make a good trend line, but I can't find any data to substantiate the claim that "virgin blood" doesn't relate to a young woman or to an actual virgin. It might be that one author or one cult used this definition of "virgin's blood," but if so, I can't find it. If /r/etymology gives you a better answer, please share it here!