Mokou changes the colors of her flame when she uses her occult(powered up), just like Mega Charizard to Mega Charizard X.
Blue flames = hotter (unless certain reactants are used)
Also, some gluons are blue.
You totally...Amazing! You're so cool!Hee heeChariza-...I don't even...what?Eeeeeee!...look like Mega Charizard X!M'haaahAww, it's nothing, really...When Mokou uses her Occult Attack, "Spontaneous Human Combustion", her flames turn blue, ya know.