When MIB told Richard to kill Jacob and said don't let him talk do you think MIB knows if Jacob talks to him Richard will listen and know he is telling the truth? Jacob had blood when Ben stabbed him and he talked to Ben. MIBLOCKE had no blood when Sayid stabbed him.
I find it odd that when Sayid let him talk ( Messed up Locke ) and stabbed him - no blood. I know MIB took over a dead guys body but Jacob has had to be true to who he was always. What if someone stabbed MIB before he took Lockes looks I bet he would still have no blood.