I did measure MGIV RC once but can't remember now if it was 3 or 4 stops attenuation through the paper. Given that the light has to bounce off the baseboard, with even white giving a lot of further attenuation, before it comes back through the paper again before hitting the emulsion from the underside, I suspect the round-trip must be worth something like 10 stops. If anyone has the time, doing a flashing test to find the shortest exposure for a change in density with and without black card under the paper might give interesting results. I suspect no detectable difference, but until the test is done, who can say for sure?
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