Here the word "blood" is a metonym for the murder. AT: "The one who avenges the murder" or "The relative seeking vengeance" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This can be stated in active form. AT: "the relative must certainly execute the accused man" or "the accused man must die" (See: [[rc://en/ta/man/translate/figs-activepassive]] and [[rc://en/ta/man/translate/figs-idiom]])