Given two heatsinks identical to each other except in color, how can the differential efficiency of thermal radiation be determined? For example, if both heatsinks are aluminum, with one anodized as black and the other with just its bare aluminum surface, then what is the percentage difference in thermal radiative efficiency?
BACKGROUND: I am an electrical engineer working mostly in test engineering. About 20 year ago, I was tasked with designing an electrical test load, which itself used fans and large heatsinks to remove heat from the test item ( a 10 KW 3-phase generator from a U.S. miliary jet ). I did ask for advice on heatsinks from co-workers who were mechanical engineers. Although we did use bare aluminum heatsinks, I never thought at the time to ask if a black-anodized radiator would be more efficient. Would the difference be $1%$ or $2%$, or $15%$?