Black rat snakes are very hardy snakes. Since you live in Florida I can only assume the humidity is roughly 80 - 100%. I seriously doubt you need one of those misters. If your snake is not having any shed problems you should be ok. During a shed(when the eyes turn blue)might I suggest a humidity box. This is as simple as a deli cup/tubberware container with a hole cut in the side big enough for the snake to slither into. Use damp,not wet,paper towel or moss. You could even leave this in there all the time if you wish but I only use it for my snakes during a shed. I am up in PA and the humidity is around 80% right now.
Also the misters would require you to chage the subtrate more often as a wet cage is bad for snakes. Overly damp/wet cages = blister disease. This is very common for captive snakes to get,especially in a damp area. Always keep the cage dry(for almost ALL snakes!) I have been told this by people who have had snakes for 25 years.