What does the phrase "purpled thy nail" refer to in this excerpt from "the flea" by john donne?

cruel and sudden, hast thou since
purpled thy nail in blood of innocence?

an injury the speaker's beloved incurred as he wooed her

the shared blood of the speaker and his beloved in the flea

the loss of the beloved's innocence symbolized by the flea

the beloved's sudden cruel treatment of the speaker