This is not a special effect. This is black fire. When you mix a sodium street light or low-pressure sodium lamp with a flame, you’ll see a dark flame thanks to the sodium and some excited electrons.
“It’s strange to think of a flame as dark because as we know flames give out light, but the sodium is absorbing the light from the lamp. That light isn’t reaching the camera and so the flame looks dark.” Natasha Simons explains the physics and chemistry behind black fire in this Royal Institution video.