In Old Norse mythology Svartalfar or Dökkalfar (black/dark elves) are some sort of subterranean anti-thesis to Ljosalfar (light elves) that may or may not be the same thing as dwarves. There's not much to go on in terms of how they are conceptualised.I thought “svartalfer” are “dark dwarfs” who live alongside the dark elves.
In classic Swedish role playing (i.e. in good old Drakar och Demoner) Svartalfer (note the slightly adapted spelling) are one of the 'goblinoid' peoples alongside 'vättar' (which is how 'goblins' were translated in the Hobbit), 'orcher' (how Orcs were translated in LoTR), 'resar' (ogres) and troll. They are lanky and sinewy, halfway between vattar and orcher, and tend to be wolfriders and brigands. This is a classic Svartalf in this sense by Nisse.