Sanji is supposed to be the "Stealth Black", his powers should have nothing to do with fire, but with invisibility. The same way Niji is the "Dengeki Blue", Dengeki means "Electric Shock", and Niji has these powers even without the Raid Suit, works the same way for the other Vinsmokes.
So there is something really odd about Diable Jambe
So there is something really odd about Diable Jambe