This week, new images surfaced of a prototype iPhone X that had a Jet Black finish (via Mr White).
Apple first used the Jet Black colour in 2016, with the iPhone 7 and 7 Plus, and was likely testing it on the iPhone X, which was released a year later.
The leaked images show a device with no markings, certification stamps or the final camera design, but otherwise looks the same to a standard iPhone X.
Jet Black on the iPhone 7 was prone to scratches and dents due to its glossy nature, a finish layer killed off on future iPhones.
The iPhone X was officially available in Space Grey and Silver. Apple later added a Gold option with the iPhone XS and XS Max in 2018.