You can probably just use matte-black paint on it. You don't need flexible paint specifically, just not a glossy-black finish. Sometimes bumper paint has just the right finish for that kind of job (wiper arms, smooth black plastic, etc.) If you do a good job of sanding, priming and prepping, it will come out really well and shouldn't be obvious. You may find yourself refinishing the rockers or something like that also, once you see how clean it looks. The good thing about matte black paint is that it is very easy to lay on as long as you don't make it run. I used to work in a body shop, and we'd touch up bumper finishes and other detail items/small areas with the can, believe it or not.