>>489956772/2
Most Pokémon Types actually don't usually work this way. Are Electric Types LITERALLY made of raw Electricity? Not really, they're Electric because they have magical electric powers. What about Sawsbuck? It's got the antlers sure, but it is not literally a plant. It is a Grass Type because it has magical power over plants.
Dark Type is kinda the opposite way where most of them literally ARE evil, but that is in no way a requirement. All that is required is for you to have "dark powers" which Absol indeed does—even though it chooses to use these powers for noble purposes. Think of Darkrai. Some media depict it as not evil. But Darkrai controls this dark force, sort of like black magic, that causes people nightmares and harms them unintentionally. It means no harm, it just has an affinity with evil forces.
Another aspect of Dark Type I didn't mention earlier is UNLUCKINESS and misfortune, like Murkrow, and DOOM like Yveltal. That is where Absol's Type comes from. It is innately tuned to cataclysmic events, and senses approaching doom. This is NOT a psychic ability, Absol is not exactly predicting the future. It is more picking up on subtle signs of upcoming disaster that others cannot detect. It is a magic ability, an affinity for what is essentially black magic. The horn is a built-in radar warning for bad luck. Not the trivial kind, Absol probably cannot detect when you will step in a puddle or even when you are personally about to die. But it senses catastrophe on a worldwide scale, the kinds of events in which many many people will die.
That's also why the horn looks like the scythe of the grim reaper - it only shows itself when many people are about to bite the dust. It is an omen. But, it is often the case that a Psychopomp is characterized as someone who is terrifying, yet the grim reaper is not there to hurt anyone. The grim reaper is just doing his job, he is not evil, her shepherds people to the afterlife so that their soul may be at peace.