I don't think Great Blue is actually part of San Diego's collection, they are wild birds. Same is true for Great Blues in many zoos, I've seen wild GB's at SDZ, SeaWorld SD, and SF for example.
Again they're often wild, as has already been noted. I've seen wild Night-Herons in several zoos.
I strongly concur with Ituri that many cases with the herons involve wild birds, signed or not. In addition to the two aforementioned species I've seen several other heron species running wild in zoos including Great Egret, Snowy Egret, and Green Heron.