It's just marketing-speak. You're more likely to buy an "Obliterator" than, say, a marker called the Daffodil.
To a certain segment of the market (young males- that's pretty much all of us, isn't it?
) "Dark" Angel sounds cooler than "Angel". That why we have guns like the "Intimidator" (which, in the hands of a 13-year-old noob isn't really all that intimidating) and the "Defiant" (named, as I understand it, after a bad-a** gunship from Star Trek