I think it's even simpler than that. Any black body that is colder than its surroundings heats up. Any black body warmer than its surroundings will cool down.
This thermos would only keep your coffee hot if you dropped it into a furnace.
This thermos would only keep your coffee hot if you dropped it into a furnace.
A perfect black surface on a spacecraft can do more than charcoal black paint, but only by a matter of degrees. If memory serves its less than a 2x multiplier between off the shelf and exotic materials. But when ounces cost thousands of dollars, you’re going to go exotic.