I'm a firm believer but physically showing proof would be difficult. You can't just run water through a black rad and expect the car to run cooler - won't happen - the temp is controlled by the thermostat. If the black rad dissipated the heat better then the thermostat would just hold the water in the block longer to maintain the temp.

The real test would be two rads side by side - one black - with same temp hot water running through them. They'd have to be in an enclosed space away from sunlight so the black wouldn't absorb heat from sunlight. You would then have to measure both the outlet temp of the water to see which had given off more heat as well as the surrounding air temp since the dissipated heat will warm the air.