In my reading of blackbody radiation I am always asked to imagine this or that body being a perfect absorber or emitter of radiation, and I am always left with the impression that a blackbody exists only as a theoretical construct. But is it? Or can one be constructed and tested?
A Blackbody is only theoretical. In other words, it is an ideal one. No such body has been observed with such a perfection in the emissivity. I think the Wiki article holds good. Its first statement:
With the help of such a body emitting radiation, we can compare the range of its frequencies with other bodies, approximating their frequencies so that it matches up with the blackbody and we can easily determine the temperature of it.
For example, the temperature of sun, molten iron, us or any other hot body can be determined by observing its spectra and then comparing them with a blackbody. The comparison matches very approximately only to a certain blackbody at a certain temperature.
Thanks to Max Planck ;-)