Actually, since Hal and the Indigo Lantern were able to fend off a BL by combining their powers and the fact that the only way to defeat the Black is to combine all the powers of the Lantern to create a second, 'Light of Creation', maybe the only way one can defeat a Black Lantern is by fully coming to terms with their emotions. If a person dies while he is at peace, like Don or Johnathan, they have come fully to terms wit their emotions and that they have no debts or regrets to anyone whereas the Martian Manhunter died in pain withering as his enemies surrounded him, so he didn't die peacefully and that's why he is a BL.