By saying that "blood will have blood", he means that the blood of a murder victim will seek out the blood of their murderer, and thus a murder will always be brought to light (see Shakespeare navigator on blood imagery).
Similarly one may ask, what does Macbeth mean by I am in blood stepped?
In this quote, "I am in blood / Stepped in so far that" Macbeth means that he has already spilled so much blood (Duncan, Banquo) that it's "too late to turn back now."
What are the three apparitions in Macbeth?
Here, Macbeth encounters three apparitions: a severed head, a bloody child, and a royal child holding a tree. Each of them respectively represents Macbeth himself, his childish naivete, and Malcolm's offensive from the Birnam Wood.
WHO says it will have blood they say blood will have blood?
Banquo's ghost coming to Macbeth causes Macbeth to act insane and irrational in front of his wife and guests, and Macbeth states that the blood he has shed will come back for revenge on him - blood will have blood.