This trick only works at night, in cities with old sodium streetlights, but it looks very badass. With just a small torch and a spray bottle, you can make the dread 'dark fire.' That's right - you will see black flame, just like in this picture. Here's how.
Some people are impressed when evil fantasy mages bring forth 'dark fire.' Some people snort, "Contradiction in terms," and move on. For once, the cynics have it all wrong! There is a way of making black flames, but it takes a certain amount of set up. For one thing, it has to be night - which is okay, because who wants to be an evil mage if it involves having to get up early in the morning? And it has to be done above the people that the mage wants to impress - which is okay, since part of being a mage is setting oneself 'above' people. That's just a matter of starting as one wishes to continue.
The first thing needed by any mage is a town with old-fashioned sodium streetlights. Sodium gives off a bright orange-yellow glow, which is meant to lend a comforting ambiance when the town's citizens are being mugged by meth-heads. The bulb of the streetlight is filled with sodium vapor. When enough energy is passed through it, it absorbs, and then re-emits orange light. That absorption is very important.
Any sodium vapor will absorb that particular wavelength of light, and then re-emit it. Unlike the streetlight, it won't channel that emission down towards the viewer. It will re-emit in a bunch of different directions, away from the eyes of common men. A dim sodium flame, placed in front of the streetlight, will absorb the light from the lamp, while re-emitting in a scattered way, and show up as 'dark fire,' an object blocking the light of the streetlight. Other people will be able to see the shadow of a sodium flame below on the street - as shown in this picture.
And sodium flame is relatively easy to make. The easiest way is with a small torch and a spray-bottle full of water with a good amount of ordinary table salt, sodium chloride, dissolved in it. Seen in regular light, the spray of the salt will make the flame glow orange. Seen in front of a sodium vapor lamp, and the flame will look dark (as long as it's not too intense, at which point it will look like just another burst of orange light). Wood or paper soaked in salt water (and dried, of course) will also look orange when burned.
And as always, the path to being a dark mage is a dangerous one, and should you have some kind of unfortunate incident when you go out at night to wave torches in the air above you, io9 will not claim responsibility. No, not even if you come at us with 'dark fire.' We know that trick.