I have dealt with a lot of supercharged GM engines both stock and modified including my 2 V-wagons and have never, not once seen what you describe.
As said, black is fuel, which you already know. When a GM engine gets "confused" they generally go full rich as a "safety". Could be anything from an injector to the MAF, MAP, O2s, or any other sensor that brings it to this "safe" mode. Even the various temperature sensors (oil or water) can do it.
Did this wonderful dealer look at any other sensor values? My bet is that somewhere there is a sensor with either a 0V or a 5V value.
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