There are a few reasons why black cats may rust. One reason is if the cat has been lying in or playing in a rusty area. Another reason may be due to certain health conditions that cause the cat's skin to break down and become susceptible to rusting. Finally, some black cats simply have a genetic predisposition to rusting. Regardless of the cause, if you notice your black cat beginning to rust, it is important to take them to the vet as soon as possible for treatment.