I just realised that a lot of Psychic-types are "book smart" — Alakazam's unrealistically high IQ, Slowking being as intelligent as award-winning scientists and having battles of wits with Oranguru*, etc — while a lot of Dark-types are "street smart". A lot of them hunt in packs that are highly co-ordinated, such as Houndour/doom, Murkrow/Honchkrow, Weavile, Mightyena, and Pawniard/Bisharp, and Weavile also have a written language. The first Dark-type moves were all weak but had added effects to compensate, which makes me wonder if the idea behind Dark-types is that they're intelligent enough to know that they're weak, so they use underhanded tactics and find strength in numbers.
*Which is related to another thing I like: starting in Gen 7, a lot of PokéDex entries mention Pokémon's relationships with other species (eg: Sandile eating Trapinch, but Flygon helping Krookodile hunt) outside of the usual pairs (eg: Zangoose and Seviper).