If his intent was black flames, then it's not really as if Sanji's powers were awakened like they were in Wano. It's also not as if Oda was leading us to any indication of Sanji not being a failure in WCI.
Queen sees the flames and is like yeah that's not normal. Which is true, as said in the SBS later. Remember that we're seeing the story from Sanji's point of view really, so none of this is explicable.
The black flames don't have to be genetic, it could be Haki but... it's not like we have a point of reference. Who else in the story is using "natural" flames that change color?