Black and tan is something that can come out in a litter as well as blue and tan. There are some historical dogs that look like they have black and tan markings but they are actually brindle (it is my understanding having never met these dogs and the pictures being hard to tell from).
It is not an accepted colour in the standard so ones that are born these colour should be sold on a spay/neuter contract. It does not mean the puppy's are not pure but I do not think it is a female that should have been bred (not just because of her colour). I am not sure what you mean by "run of the mill" Cane Corso but for a quality Cane Corso pet you are usually looking anywhere from 1500.00 to 2000.00 depending on the region you live in.
If you decide to go with this breeder make sure you check into health screens of the parents as like most other large breeds they are prone to certain conditions like Hip dysplasia, elbow dysplasia, Cherry Eye, etc. There are a lot of skiddish/nervous dogs being bred so be sure of the temperament of the dog especially if you have kids or people around often.