Apart from contrast issues which I mentioned a lot of times in 'KK please read' thread, there's a fundamental problem with the black flaming C. Black is the color of something that's burnt therefore every flaming logo in black simply loses its impact.
As for the contrast, sometimes a picture is worth a thousand words so this is what, in my opinion Flames are doing to their logo:
Back to normal:
You'll never achieve the same contrast and impact with a black logo on red, even if its shape is perfect.
The interesting thing here is Lego comparison as in black it suffers from the same blending issue as the black flaming C. Clearly, the effect of double outline is lost as white and yellow blend together in small scale (or seen from a distance).