This particular quote comes from Hale in Act IV.  It is at this point where Hale has understood he was manipulated by those in the position of power in Salem.  He is filled with guilt for his role in condemning those to death under false pretenses.  To this end, his speaking to Elizabeth Proctor is one where he attempts to convince her to convince her husband to not go ahead with his planned execution.  He hopes Proctor will lie in order to live.  For Hale, admitting to such a lie in the name of something greater is a worthy transgression.  Hale sees life as the most important gift that God can give to a human being. The fact that he was used, or duped, to take that gift away causes him great guilt.  The "blood" that is on his "head" is reflective of those who have died or been condemned to die under dubious circumstances or manipulated charges.  Hale wishes to stop this and believes that in saving the lives of those who are condemned to die he can "get right" with his own convictions.  In Hale's belief that lying to live is important, he argues that his own guilt compels him to embrace such a position.  This "blood" is symbolic of those who have been sentenced to die with his unknowing help.