While maybe also being in the ‘dark’ category, I’ve never been one to really use it. Experimentation, torture, rot, disease, general agony. Though dark can be beyond evil as evil is subjective. The original idea of the Scarlet Crusade was was a darker version of the paladin, zealots, uncompromising and radical who tortured, maimed, and pillaged in the name of the light, a “good” force, for the betterment of their conditions.
What I’ve seen meant by it most is one of two things:
A preference or openness for violence, torture, pain, suffering, manipulation, etc in the stories and campaigns they participate in. Think of your very best grim dark, and thats what they want. That does not necessarily mean that your character is dark, you could strive to be the moral conscious in such an arch, but that is what the story holds. Though the latter is fairly rare.
The other is the psychologically ‘dark’ character, those who want to play dark characters, actively pursue and create dark themes, and who’s entire character/description generally revolves around them. The Death Knight who, by definition of being a Death Knight, needs to inflict pain and suffering to survive. The plague doctor, Scarlett torturer, raiders and pillagers.
It can also just work as a TW. “Hey, this person is pretty rough and can have some concerning themes. Perhaps read on at your own risk.”
In all, dark can be for you. You enjoy being around those themes and depictions. I would perhaps suggest “Dark themes welcomed,” if anything at all, if you really want to add it to your TRP