Most bioluminescent ocean dwellers produce blue-green light, because its wavelengths travel farthest in the water. Because this applies to the sun's light too, marine life tends to have eyes that are only sensitive to a small range of blue-green light. Black Dragonfish, as well as the rest of the the Malacosteid family, produces deep red light as well, and has a novel way of sensing its reflection.
Using red light is obviously advantageous because of other fish's inability to see it. That is, if the Black Dragonfish is using its red bioluminescence to find prey, the prey won't be able to see it coming and escape. While the Black Dragonfish is able to produce the usual blue-green light through a separately evolved organ, it's theorized that it's only used for mating and warning, while the red light is used for hunting.
Production of the red light is pretty interesting, and involves fluorescence, sort of like a natural black light tube (only producing very long wavelength light instead of very short). Light produced in the Dragonfish's photophore, the light-bearing organ, is of a pretty short wavelength, sort of a bright orange. It is absorbed by fluorescent pigments inside the photophore, and re-emitted at a much longer, and thus redder, wavelength. Additionally, there is a filter over the photophore that lets only the longest waves through, creating the Dragonfish's near infra-red luminescence.
The way the Black Dragonfish senses reflected red light is also cool. Its eyes are like those of most other fish, and only able to see the blue-green wavelengths of light. Its eyes use a visual pigment chemically related to chlorophyll, which turns the red photons into electricity that directly stimulates the blue-green sensitive nerves. The special pigment is commonly referred to as an antenna pigment because of the nature of its operation, turning the signal into electricity instead of a chemical neurotransmitter of some kind.