Except "black body" here is a technical term, and doesn't really correlate with "an object that's black". You could, for example, design something that absorb

Except "black body" here is a technical term, and doesn't really correlate with "an object that's black". You could, for example, design something that absorbed well in the UV/visible/near infrared where sunlight energy peaks (sat, 250-1000nm) but has low emissive power in the far infrared of the ambient Martian surface temperature at or below the target temperature (say 2-100μm). Thus it would absorb a lot in the day but radiate far less in the night, all with no need for electronics, etc. The wind powered heaters (as I recall) had a similar issue; if the wind wasn't harvested to run heaters, it would have dissipated through friction with the ground, generating exactly the same amount of heat.