The whole question of PRR DGLE or "Brunswick Green" is one that will be debated to the end of time. It is such a challenging question that the PRRT&HS has a committee just on colors and they are still discussing the correct drift cards for this color.
While no PRR units were painted totally black, when fresh out of the shops they were nearly black. I don't remember the formulas, but it was something like a few drops of green into gallons of black paint. From what I understand the whole model version in pure black comes from the restoration of GG1 4935 in 1977 by "Friends of the GG1" and Amtrak. That unit was regularly referred to as "Blackjack" due to how dark the DGLE was after a fresh repaint. AHM did a commemorative version that was painted black in the 70s and built by Rivarossi. I believe that is where Williams got the idea to paint theirs black.
Overall, the Williams version while it doesn't have any green in it, may be more accurate then a lot of green PRR locomotives that have been produced over the years.
