Hi Nominee, Neal is correct.This idiom is thought to have come from the world of boxing. The expression is also used metaphorically to mean, "to score a victory over someone"
eg In the debate, it was the visitors who drew first blood.
We were hoping to draw first blood. Unfortunately, we didn't.
"Draw blood" has a third meaning as well. When you "draw blood" you succeed in hurting someone's feelings.eg The boy's scathing remarks clearly drew blood.
eg Sam's remarks were intended to draw blood.