What is a blackbody?
A blackbody is an object which absorbs all incident radiations it receives whatever the wavelength or direction and re-emits all these absorbed radiations. An important property of blackbodies is that the re-emitted energy level only depends on the temperature of the blackbody.
For a prescribed temperature and wavelength, no surface can emit more energy than a blackbody.
A blackbody is then an optical reference source, though a theoretical device.