Right, but that means that stealth costs mass (weight) and means less satellites per launch. So that means making them less visible costs more money.
For non spy satellites where stealth isn't one of the top goals, that might mean people don't do it.
Also, typically one does not simply ask a surveillance agency how they do things. They would probably respond with something like, "NO SUCH PERSON AT THIS ADDRESS, RETURN TO SENDER."
From what i read they are going with "partially painted black" Makes sense to only have the underside facing earth black. Also the satellites underside is only exposed to the sun with an steep angle because most of the time when it would be exposed to sunlight it's going to be in earths shadow. Reducing the extra heating even more..
Oops that's classified. But probably such satellites need a dedicated cooling system.
It's never as easy as, just paint it black and hope for the best.