In some instances, like bamboo fly rods, you will see a lot of blued metal parts such as the ferrules, winding check, & maybe even the reel seat metal. On synthetic rods, it's not uncommon to have all black line guides & reel seat metal (coated at the factory through a special, commercial process) with only the winding check blued.
Whenever a rod maker blues any metal parts it's important that they get them to match as closely in color as the other darkened parts on the rod. This will give the rod a look of uniformity & you won't have any particular metal looking way different than the others.
In the photo below, I'm tasked with bluing this winding check to match this stripping guide. This is what it looked like to begin: