Casual viewers dubbed it because of the way the starship dims and then blackens, turning so black it’s silhouette is visible against the black, starry space. Then it just fades away.
It is the diametric opposite of the previous Faster-Than-Light drive. Standard FTL involved using massive quantities of energy to literally rip apart the woof and warp of space-time. For a brief time, all of Hell’s Fury is released. Once things have calmed down a bit, its mostly safe for travel with a properly equipped starship.
Shadow Warp, once scientists grasped the concept, was simple by comparison. Instead of ripping space-time apart, one simply slips between it. Or, at least, that’s the easiest description they offer without delving into 7th-dimensional physics.
The problem is that it’s horribly inaccurate. Computer guidance barely avoids warping inside of celestial bodies micro-seconds before arrival and you could be anywhere within 30 parsecs of where you intended.