>>47833342I always figured it was something akin to "emotional damage", since underhanded techniques can be seen as inducing stress and desperation, and even Dark Pulse's description states how it's imbued with "dark thoughts".
That said, while you can explain why there are dark attacks, it's harder to explain how there are dark Pokémon. What's different with their bodies from other types? How does their dark disposition affect their resistances? It only really works since there are other types who are described as just and/or valiant types (bug and fighting), but in any other setting it becomes really ambiguous.