bote wrote:*slaps his forehead*

That theory about oxygen being necessary to light the fire was invented by cavemen!
Hm...not sure if this is a joke or a statement or even whether it's an agreement or a jab. I'll do my usual rebuttal anyway. ;p
Fire can't 'light' in the absence of oxygen. If you take a tank of gasoline (highly burnable) and stick a flamethrower inside that tank so that the gasoline surrounds it - and then try to 'light' it. The fire won't catch. For a fire you need something to burn and oxygen. It's the gasoline fumes that spark most fires, not the liquid itself (which is why those fires burn on top and not through the liquid). In a normal situation - like space, or the inside of a gasoline tank, no spark will catch (no oxygen) so it won't happen. The only explosions in space are when some burnable gases ignite - and even then they tend to implode from the pressure and burn themselves out (eating up whatever pocket of air there is until the fire can't 'live' anymore).
But this is anime - the fire in question wouldn't be *starting* in space - it never does in the anime (meaning it doesn't start where Hiei is). It starts in the Makai, travels into whatever world he's at (the black pilars when he's in the human 'upper' world) and then feeds off itself and his youki. That doesn't defy physics (the rules we follow when it comes to fire) because the flame itself carries with it an oxygen source - all that's left is the 'fuel' to keep it burning - and that comes in the form of Hiei's youki. Just think of his youki as gasoline fumes - that provides the fuel once the spark catches enough oxygen to become flame. And since Hiei can continue calling black flame (complete with its own Makai mix as the oxygen source), it will continue to burn until the fuel gives out (his youki) or he runs out of things to burn (other people, clothing, anything flamable).