Black surfaces are good at absorbing AND radiating heat. I think that’s why the SR-71 was painted black. The amount of heat it got from absorbing the sun’s rays probably paled in comparison to the heat it radiated from all the friction as it flew.
From what I understand, the SR-71's black paint did radiate thermal energy, but also made the plane harder to see in visible light at night, and on radar. It was a RAM coating impregnated with iron to reduce the radar cross section. The characteristic chines along the edge of the fuselage were also meant to reduce the radar cross section.
