In the Adventures of Tom Sawyer, Tom and Huck go into the graveyard with the dead cat at midnight. It is supposed to take away warts. This is how they end up seeing the murder.
The other superstition is that murdered bodies will bleed in the presence of their murderers.
A pact signed in blood is a binding agreement and if you break it, you die. Tom and Huck sign one to promsie that they won't tell who murdered the Doc.