You can be certain you won't ever get the Blackvenom Blade. And that's OK.
While I cannot speak for the scaling at level 20 versus 24 I can tell you from experience that play style and skill matters more than BiS. If you do decide to stay at 20 you've got plenty of options for daggers.
Wicked Dagger,
Doomspike, and
Dissector are up for grabs. It depends on if you want to be an agility stacking glass canon or balance surviving with some stamina. And the PvP dagger isn't a bad choice either. When you see how hard this bracket hits you might want a little extra resilience.
Also, at 24 you can get the sick-nasty
Bangalash's Claw.
Personally, I rolled to 24 because I wanted the extra interrupt with Gouge and the best dagger I could reasonably get (should have farmed the BvB a long time ago).
Oh, and I strongly recommend rolling Assassination with double Elemental Force. Ambush, Mutilate, Mutilate, if they're still wiggling, Envenom. If they managed to get distance between you, stand there and Throw daggers. I cannot even tell you how many hunters, mages, and druids I've finished off this way. Every hit means EF might proc, including Throw. Now if only Blizzard let us use Throw while moving...
Masseur @ Darkspear - Community - World of Warcraft