Our new 2 layer “After Dark” service features black FR-4 substrate with clear soldermask to show off all those beautiful copper traces.
Our “After Dark” service is the same cost as our 2 layer purple PCBs: $5 per square inch, which includes three copies of your design. For example, a 2 square inch board would cost $10 and you’d get three copies of your board. You can order as many copies as you want, as long as they’re in multiples of three.

Aaron (@TwinkleTwinkie) wrote a nice post on Hackaday.io about making:
PCB Art with OSHPark After Dark
Here are some examples of “After Dark” from Twitter: