With how prominent Haki is in the New World, I used to think that Oda will go to the half-Haki half-elemental route for Sanji. This allows him to imbue Haki to his flame, which will color his fire to black. Think Luffy's black fist or Zoro's black blade. Eventually, Black Leg Sanji moniker will make sense again.
But Germa's introduction steer Sanji's development to the science route, just like Franky Usopp and Nami.
Since Oda take the science route with Sanji, I doubt black flame will be a thing for him, because it is scientifically impossible.
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Unrelated, but peak Sanji in my fanon is white flame with white lightning on the right leg (science) and black flame with black lightning on the left leg (haki), while wielding supreme grade knife in the right hand (ala Enies Lobby) and pistol on the left hand (ala Alabasta). Chuuni as fuck, but one can dreams.