Most everytime I see this question... it's referring to electrics. I think that blues tone on an acoustic pretty much comes from the fingers... Where as electrics you have to have the right tone as far as pickups, effects, and amp settings go... With acoustics... it's just wood and metal. Not much to it. If you're looking for a deeper tone... Look for guitars that are made of body woods like Koa, Mahogany, or something similar... Nato is basically lower end mahogany... Good brands that produce a deep tone are Martins, Takamine (can produce bright tones if you get the right model/wood) and some Gibsons. I'm not too familiar with Alvarez, Guild, and some of the other acoustic brands...

String guage can play a big part in what your tone is with an acoustic guitar... as well as what your strings are made of and what brand they are. For a deep bluesy tone I suggest a medium to heavy set of strings... I suggest Elixers... however, Elixers generally brighten up the tone so check out some D'Addarios or Martin Phospher Bronzes.