Blackbody Radiation:
Heat loss by radiation was discussed in PHYS 201 in the Thermo part.
The light from a blackbody is light that comes solely from the object itself rather than being reflected from some other source.
A good way of making a blackbody is to force reflected light to make lots of reflections: inside a bottle with a small opening. [Even if light loses only 5% on each reflection, after 25 reflections the light is down to 28% of its original intensity.]