You indicated the primary test ok. Is it around 1.2 ohms or less? While it is true that B+ is supplied to the coil's primary and the ground is turned on by the ignition modules circuit. It sparks when shut off thru mutual induction into the secondary side of the coil. They are insulated apart from each other and while B+ is available while the engine is running, it remains turned on to create a magnetic field. There would be no reason for the secondary to open unless an internal insulation issue. Some coils (Denso) have a diode that will not allow ohms testing from the spark plug end to the ground side, either coil frame to ground when bolted down or thru a wire in the connector. I was taught to use the GM spark tester ST-125 (S=spark, T=tester, 1- version 1, 25= 25k air gap) With coil grounded and plugged in, the tester is plug into the spark plug wire led. Spark or no spark. This test remains the same for coil over, coil near on new products. With mutual induction, even with a bad wire or plug, there is no reasonable answer for burning out the secondary side of the coil. Key on, engine off, using a digital multi-meter (DMM) test for ground from block, battery using ohms. Reading should be 0.400 ohms or less. Clean grounds at battery, engine, frame and ECM G100 on the engine from the harness. B+ should be battery available depending on running or not, but the same as system voltage. Re-check how you are testing this coil and try moving a known good coil to that location. I am not familiar with these engines as i worked at the Buick dealer when these where in service, but if you have multiple coils, move them around. Since the secondary produces very high voltage with very little amperage (>0.005 amps), only internal insulation can cause the wire wound in the secondary could short out and go open. Best of luck, let us know what you find and photos would be nice also.