Long time ago the coupler pins were chemically blackened.
Then they stopped that practice for quite some time (the pins were copper color), Now, as others have mentioned, they again started chemically blackening the pins. I'm glad they started doing it again.
EDIT: The above applies to couplers included on their ready-to-run rolling stock. Like jagged ben stated, the individual couplers always had black (never bare copper-color) trip pins. Well, at least in the last 40 years.