I would agree that the likelihood is the number of black dogs is indicative of the fact that black is a very commonly expressed gene- it is a dominant gene for one, and is present in quite a lot of breeds. I do also agree that for shelters not pulling dogs from overseas or bringing dogs in from other areas, the surrendered dog population is pretty reflective of the breeds/types common in the area.
In the area I am in (norther CA), I actually don't see an extraordinary number of black dogs in shelters, and certainly don't see more owner surrendered black dogs than other colors.
Most shelters I've interacted with are very open with reasons for owner surrenders, and if its not listed on the kennel card they will usually disclose if asked. The majority of surrenders around me tend to be because the owner is moving and can't find housing, or the owner doesn't have the time for the dog. My understanding is those tend to be pretty prevalent reasons for most shelters.