If the game plays blind, the failure can be in a number of places. If the color DAC's 74HCT273 chips have failed, then nothing will drive the video DAC. However, if the game is playing correctly, then it means the palette RAM is working, and the '245s controlling palette RAM address mastery haven't failed; the startup test will catch this problem. It is possible that the blanking signal (going to pin 1 of the '273s) is stuck, blanking the whole screen.
It is also possible that the GAL16V8 at position U33 named "TWIN-CAM" has failed, which is responsible for muxing text and GP9001 video output (and generating the blanking signal mentioned above). Check the page on the Raizing "TWIN-CAM" U33 for details.