White objects are not perfectly reflective and whiteness is not so much a function of reflecting them well, but reflecting them relatively uniformly in the visual range. Not every white object reflects UV.
Black plastic lasts longer because 1) the UV is absorbed by the pigment, not the polymer and 2) the pigment limits UV penetration to the surface.
> I thought black objects absorb all wavelengths, and white objects reflect them all away.
The point about a black piece of plastic is that the energy is absorbed at the surface -- it can't penetrate the plastic to any significant depth. That means the majority of the plastic is protected from the ultraviolet wavelengths that would destroy it.
> Real world materials are rarely ideal black bodies (or ideal reflectors).
That's true, but it's also true that all objects eventually reach thermal equilibrium with their surroundings. Were this not the case, it would violate the law of energy conservation, and that's a law that's never broken.
In that sense, most objects, solids and plasmas in particular, are reasonable black bodies once they achieve equilibrium. Some of them show spectral lines -- emission, absorption or both -- but in most cases those lines appear on top of a blackbody curve. The sun, for example -- perhaps surprisingly, it exhibits a classic blackbody spectrum appropriate to its surface temperature, with many spectral lines contributed primarily by its comparatively low-pressure atmosphere (http://commons.wikimedia.org/wiki/File:Solar_Spectrum.png).
The state of matter least like a black body is a low-pressure molecular cloud in space, which often shows only emission and absorption lines, no classic blackbody curve. The lowest pressure gases show the narrowest spectral lines, which is how we can estimate the pressure of a remote cloud of gas illuminated by distant energy sources.
Sure, but to the extent they're imperfect radiation absorbers, they're also imperfectly black. It's like how we can discuss pi and circles even though the real world has no perfect circles.
