it's just painted black I guess?

http://www.calcuseum.com/SCRAPBOOK/BONUS/48558/1.htm

https://youtu.be/MsiOue7vP1k?t=175

https://youtu.be/oSoYenJ7mkY?t=169

as seen in the game: https://youtu.be/9WcD7ZDTYyo?t=84

notice the pattern of the arrow keys, the up and down keys to the right of the main arrow keys, and the top row is somewhat visible too but much darker, you can see the last 3 keys on the top row are spaced apart from the rest