Unlike the Flying Nimbus, the Dark Nimbus can carry people of an impure nature. Much like the Flying Nimbus, its primary function is a mode of transportation for one or more people. Presumably, it can also be used to carry cargo if the need arises. It looks exactly like the Flying Nimbus, but instead of gold, it is dark purple in color. Korin can also change the Dark Nimbus from a solid to pure gas in order to make its cargo or passenger(s) fall straight through, much like someone of impure nature attempting to ride the Flying Nimbus.
In a filler portion of Dragon Ball, the Dark Nimbus was lent to Mercenary Tao by Korin after he had climbed the Korin Tower and drank Korin's Sacred Water (which was actually just ordinary rain water). This was done to ensure that Tao would gain no further strength on his way down as the true enhancement to one's self from Korin's Tower comes from the climb up and down Korin Tower as well as the training with Korin himself, in between. Half-way down, Korin let the cloud drop Tao, thus causing the mercenary to crash into the ground at high velocity, taking damage to start off his rematch with Goku. This is the only appearance of the cloud in the series. It is unknown whether or not there is a huge cloud containing multiple Dark Nimbus, like there is with the Flying Nimbus.
The Dark Nimbus is never used or mentioned in the Dragon Ball manga, but the regular Flying Nimbus appears with purple color on several chapter title pages, on an image the series' author Akira Toriyama made for the 1993 Weekly Shōnen Jump #2 (also featured in Daizenshuu 1), as well as on the cover of one of the chapters of Nekomajin.
Video game appearances
Krillin also uses this item during the flying stages of his Story Mode in Dragon Ball: Advanced Adventure (due to the fact Krillin is "impure"). As Krillin's Story Mode has no actual storyline or cut-scenes, it's never explained how he obtained the cloud. In the English releases of the game, it is called the "Black Nimbus".