What you're saying only depends on whether the dub was specifically created for TV or not.Originally Posted by Zinobi
Take Trigun, Cowboy Bebop, or FLCL for example. All of these already had an existing dub before there was even any talk about airing them on [AS]. The same goes for Evangelion (and probably a few others), yet it's dub is widely regarded as lower-quality than the others I mentioned. Dubbing quality usually has nothing to do with time-slot placement.
Shows on Toonami follow the same pattern. There have been good dubs and bad dubs regardless of age consideration. Whether or not you actually like DBZ, I'd argue that its FUNimation dub is vastly superior to the original Japanese dub, yet by its nature, it is a "kid's show" so it will never really be taken seriously (you should probably take this into account when judging any 'kid's show" dub, actually). On the other hand, Outlaw Star also had its dub (again, pretty well-accepted) even before Adult Swim existed. This made-for-tv dubbing seems to be only a recent trend.
You bring up a good point with Naruto and Bleach, because both of these have a made-for-tv dub. But Naruto was never really was a "mature", or "young-adult" anime...so it's hard to guess how much it could have changed if placed on Adult Swim instead of Toonami. They're both dubbed by VIZ, so you can't really argue that the voice acting would improve too much, since they'd be using the same people.
In any case, it looks like Blood+ will be a made-for-Adult Swim dub, and it's already got a few VAs listed (Crispin Freeman and Kari Wahlgren), both of whom have never "butchered" a single role I know of. So that's good news.