The Dark Nimbus is a magical cloud like the Flying Nimbus, however it is dark in color and can carry people of an impure nature.
Like it's more widely known counterpart the Flying Nimbus, the Dark Nimbus serves as speedy transportation through the air. It appears in a filler portion of the Dragon Ball anime and makes no appearance in the original manga. Korin gives the Dark Nimbus to Mercenary Tao after he climbs Korin Tower. Korin can remotely change the cloud to cause passengers to fall through it, which he does shortly after giving it to Tao, causing him to take damage from a great fall before his battle with Goku.
Other than being a dark blue/purple color, the Dark Nimbus is identical to the Flying Nimbus.