LOL! Don't listen to Boltripper!
It is a dark nickel coating on the case. They are reloadable, but the coating can flake off and scratch your dies. It's mainly cosmetic.
If it's nickel plating, it can also dull your cutter VERY fast when you trim them. About 20 cases was all it took for me to dull a Redding $18 cutter blade until it was unusable for trimming .308 caliber cases.
If you don't shoot a lot, reload them once maybe twice. Once they start to flake and need a trim, toss 'em. If you plan on shooting a bunch, then I'd start fresh with some good quality brass cases (non nickel coated).
Neck size then and you are ready to go. You can also neck and bump the case down .001-002 and then use them. I have at least 6 reloads on mine so far and still going strong.
The finish is nickel plated and coated with teflon. Makes shelf life a bit longer than regular brass cases (ie less prone to tarnishing) and the teflon aids in feeding and extraction. The black finish is so you lose your brass after doing a drive-by at 12 o'clock at night.
Interesting. I have a couple hundred 1x fired nickle plated Winchester cases. I trimmed about 40 and didnt note any dulling of my cutter. I planned on reloading them but dont want to mess up my dies or cutter. Maybe I will just reload them and give them to a buddy for his FAL.