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.

Wind is caused by thermal gradients though, right? So you’re still moving heat from one place to another.

Random placement, and you’d be correct. Not random placement would be a grey area.