the other kishimoto used black flames in 666 satan and i believe that was before it was used in naruto
Thanks ^^. I haven't heard of that one before actually.
OT: Been a minute since I created this thread :p but I think there are quite a few possibilities for this.
1.) Sanji's red flames could be a partial awakening of sorts of his gene manipulation and a full awakening could see him develop black flames. Reasoning for this could be that Sanji's body couldn't withstand the black flames without developing his exoskeleton first so a full awakening with the exoskeleton will enable him to be able use it.
2.) Using the same logic as above of Sanji's body not being able to withstand black flames hence the reason why he couldn't use it, Sanji now posses the RS that just so happens to be fire resistant could potentially allow him to be able to use it.
3.) Using the concept of Haki flow to infuse his flames with armament.
No.3 is the least convincing for me as I'm not really sure Haki flow could be used to blacken flames (i.e. I mean would it be black or would it be invisible if Sanji incorporates), tho I find the concept quite interesting. Moreover, Haki flow seems to be a semi-advanced application of CoA which isn't Sanji's specialty and Sanji hasn't really done any additional CoA training post-ts like Luffy for example, as far as we know.
I'm personally leaning towards 1.) tho ^^.
Also y'all should also keep in mind that the vinz are capable of using special abilities, none of which conform to the conventional colors we are used to. Ichiji's lights/sparks are "red", Niji's electricity is "blue", Reiju's poison is "pink", so if fire is indeed Sanji's genetic ability it's very likely that it won't be the conventional color for fire so we could have something like black.