Okay… here’s an idea I had a long time ago: it does not emit light, it does the opposite.
Hear me out: light consists of photons, they are something like tiny concentrated energy bolts, they carry both light and heat at the same time (ever seen lightbulbs burn hot?) Heat is light and light is heat.
So if we are looking at antifire that consumes energy (heat), you could also make it consume light… Cold is darkness and darkness is cold. This thing would look exactly like flames, but they originate away from the centre of the reaction and converge towards it, they are black and get blacker towards the centre. The whole thing emits darkness (imagine that it attracts light into itself and redirects it from hitting surfaces around). Then consumption. In a fire, you see matter getting broken down and energy is released. In this antifire thing, you could imagine that the energy it consumes is used to bunch up matter. In other words, it would consume smaller particles to create larger bundles of matter (think of how wood turns to ash, but in reverse — ash turns to wood). Maybe it produces fibers or whatever that get dissolved in liquid air (it’s cold, right?) and this flows around from under the antifire in streams. As the air evaporates when it gets farther from the source of cold, the fibers are deposited, so instead of having a candle that burns down, you get antifire which builds its own candle up underneath/around it.
Not sure about starting materials. You may have to create some fictional ore that can start this process with an “antispark” or such. To stop this process, you’d probably need vacuum, so that it runs out of “raw resources” from which to build its product material. Then it would have nowhere to deposit the energy it collects and potentially go out with an explosion, releasing all the excess energy. As a twist, before going out, you can make it reuse its “product” material to make heavier/more complex material. For example, if it combines hydrogen into heavier atoms, it can go all the way to Ununoctium until it runs out of matter. Its preferred product would obviously be the simplest it can create from the matter/energy at hand.
Source: an art project I did in high school was a drawing of a world in which light and dark are switched, so wherever there is conventionally light there is now shadow and vice versa. This idea kept coming back to me ever since until I formed it into the concept I described above.