The 642 comes from the J frame (small frame) family of S&W revolvers. There is a "black" version of the 642 - it's the 442. The gun is not entirely blued, the frame is aluminum so it's black anodized and the cylinder and barrel are carbon steel, they may have matte blued finish on them. I carry a 642 everyday, since the cylinder and barrel are stainless and the frame is anodized clear aluminum, the finish helps protect against rust and perspiration. Most people who carry the 442 eventually will get some rust on them, a lot of owners send them off to get some version of polymer coating to protect against corrosion.
There are J frames that have external hammers, such as the 36 and 37 (blued) and 60 which is entirely stainless steel.
There are others that either have external or internal hammers that are made from extra lightweight strong alloy such as strongtium or scandium. If you've never shot a small revolver, best stay away from these extra lightweight beasts.
Most people carry these little gems in pocket holsters or ankle rigs. They're nice to have when you're not packing any heat
