This one was odd in the sense that I was having the most issues during gameplay, but not on the title screen. There was occasionally flickering and artifacts on the title screen where it was all text objects, but once a background was introduced (such as during gameplay) I ran into issues.

With the image being so dark, my kneejerk reaction was to immediately look at the PROMs. I figured that they weren't properly being enabled. Looking at the schematics I started with the background PROM, I could see that it was being enabled, but looking at the input signal on 5D pin 4, HBD, the signal was looking very sketchy. There was pulsing but it wasn't really going "low". It was stuck around 5V.

Tracing that back, that signal was generated on the video board by a 74LS00 NAND gate. Poking that I could see that the inputs were valid but the output was registering high.

Input:

Output (stuck high):

I had found a problem. One thing to note is that the chip was made by Fujitsu, those seem to have held up rather poorly because I've noticed a higher rate of failure amoung older Fujitsu 74 series ICs. Pulling it out of circuit I verified that it was bad.

After replacing it, that made the graphics normal again. No more dark graphics and no more artifacts.