How do you make a black PCB instead of green.?

The answer depends on what you want to be black, the solder mask or the actual PCB material. The typical copper clad gl/epoxy PCB material is inherently a pale green color which has nothing to do with the etching process. The board material can be made different colors by using pigmented epoxy resin. I have personally seen unmasked boards in red, blue and black, but It might be hard to find and would probably be an extra cost option. If the green your referring to is a shiny emerald green what your seeing is a translucent, colored ink called solder mask ink. It is screen printed onto the board to repel solder every where except the points where the connections are made. I have seen black solder mask ink on boards before. If you ask at your board house of choice it may be a low or no cost option. If it's too shiny for your application you can spray it with a matte clear. This might give you the look you want without having to buy custom made carbon fiber copper clad! That would be cool though... Hmmm.