I highly suggest black milksnakes. They are the largest of the milksnakes and pretty easy going once they are adults. Once they are full grown they are jet black. Mine is about a year and a half old and really you can only still see the red on his belly.
This is not my guy but gives you an idea of size.
Mine is still young and around 3 feet long. 5-6 feet is what to expect from these guys. They are neat snakes and do get quite a bit bigger than Black king snakes.