Black 4.0 is the blackest paint you can buy

Yep. When I worked for a solar thermal company that turned water into turbine-ready superheated steam [1] using mirrors and black pipes, a lot of science went into the paint. Black is the best colour for both absorbing and emitting radiant heat. They managed a formulation that was - by the standard of black paint - a decent heat absorber, but also a very low-quality heat emitter, so any heat absorbed tended to stay in the pipes instead of radiate away.

[1] 450 C / 840 F, with production measured in tons / hour.

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