Since nobody acted
based on the wrong cards (they were all-in preflop), I attempt to reconstruct the proper board the best we can figure out.
I'm bothered by the uncertainty here, since we don't want the player with AA to be lying about the flop being from the blue deck. If you're sure the dealer used the wrong deck, though, deal the board from the correct one.
If any action had taken place based on the blue-backed cards, the hand is voided, all chips go back and we redeal:
3. If a card with a different color back appears during a hand, all action is void and all chips in the pot are returned to the respective bettors. If a card with a different color back is discovered in the stub, all action stands.
I'm adding the part that a wrong-color card only voids the hand if someone acts on it, just to keep the dealer from having a cheap way to exit the hand by grabbing the wrong deck -- other players should have a chance to say "stop, wrong deck" before things get irreparably fouled. In a casino, one wrong-deck card hitting the board probably would void the hand, but there we don't have dealer/players, and the way decks are handled there's much less opportunity to get them intermingled.
Anyway, voiding the hand is a fine, but the small blind should get his chips back too.
A couple dealing procedures could have helped avoid this. Ask players not to set the next hand's deck next to the dealer (a bigblind shuffle helps this a lot). Also the dealer should pick up the board and the shuffle should start right away after the hand ends, even if tables are moving. Discovering this sooner would have made it much easier to fix. Leaving the board out when people aren't all paying attention can lead to a lot of uncertainty, which breeds suspicion.