black surfaces have a higher radiation coefficient (alfa I believe) then shiny/reflective serfaces. the perfect "black" radiation body has a coefficient of 1 (it will absord 100% of energy radiated onto it), while a polished (mirrorlike) metal surface can have alfa of 0,1. black surfaces absorb heat (transfered as radiation energy) faster also, but that's not a problem here, since the heatsink will always be hotter then the surrounding.
As for the paint layer acting as insulator - thats correct, however, there is also a slight increase in effective surface (the thickness of the paint makes the heatsink slightly bigger) and the effect of increased radiation of heat to the surroundings, and those two combined can have an overall positive effect - so more heat is lost then with a shiny pure aluminum heatsink.
This is why for example thin copper wires transferring high current (Ampere, not high voltage) if "insulated" with a layer of plastic, are less hot then bare wires! There is however an optimum here between increase in surface area, and increase of heat transfer resistance (because of the insulation). Same goes for the insulation of pipes with hot water in cold surroundings - if too thick layer of insulation is placed, more heat can be lost actually
Of course it is important to place the pc in a surroundings that has no surfaces hotter then the heatsink - because those radiate heat as well, and this heat will be absorbed by the heatsink of the ps. So dont place the pc in front of the radiator, or even in straight line "of sight" of a radiator. better to have a barrier between those two. This barrier better not be reflective either (polished metal for example) or the heat radiated from the heatsink will be reflected back
So an insulator is best - like a furniture from wood or plastic . Also keep the pc away from the window if the sun shines (same reasons here) as window glass is transparent for infrared energy.