According a 1788 printing of The Works of Aristotle (which wasn't really by Aristotle) "virgin blood" means the blood of a virgin.
A third cause of natural barrenness is, the letting of virgins' blood in the arm before their courses come down
The way to prevent this is to let no Virgin blood in the arm before her course' come down well
See also this 1784 printing and 1791 printing.
Also, the 1827 The Seven Tragedies of Æschylus, Literally Translated Into English Prose says:
if I shall slay my child, the idol of my house, staining a father's hands beside the altar with the streams of her virgin blood...the blood of the virgin ... her virgin life