Yeah, but its not much of a dream darkroom when you wear waders! :-)
The raised floor looks to be the best bet, but I wouldn't use Styrofoam, as that will compress and age. It would be better to use a non-compressible plastic product. Maybe use PVC pipe ends instead under the studs. Also remember to ventilate the space under the floor. Forced air would be best, as you wouldn't want water vapor to mess things up. Also you may occasionally want to clean the concrete, so factor in being able to pull up sections of the floor for cleaning.