In my understanding, these terms can be used interchangeably, both describing the same beings, elves that live in the ground: Dwarves.
Now it is worth noting that the term Svartalfar ( swart or swarthy elves) has been used frequently in a few historical documents; Dokkalfar (dark or black elves) is pretty much used exclusively by Snorri Sturleson in The Eddas.
While the Eddas are arguably the most popular collections of Nordic myths, Sturleson's work has been under fire by Heathens and historians alike; being accused of many historical inaccuracies, such as:
And most importantly, creative liberties on the authors part. Most notably the "Christianizing" of many Norse gods and ideas, such as the addition of the duality of good and evil, and the demonizing of Loki ( making him more like the evil Christian Devil instead of the mischievous trickster).
Because of these reasons the Eddas are usually not taken at face value as an accurate depiction of Norse religious culture. So on some speculation, it could just be either a translation issue in his writing, or in my opinion a creative liberty used in the description of the Dwarves.