Blackbodies are called "blackbodies" in the context of blackbody radiation because they are idealized objects that absorb all incident electromagnetic radiation without reflecting or transmitting any of it. In other words, they appear perfectly black.
The term "blackbody" was first introduced by the German physicist Gustav Kirchhoff in the 19th century. Kirchhoff observed that when an object is heated, it emits radiation over a broad range of wavelengths. He conceptualized an idealized object that absorbs all incident radiation, regardless of wavelength, and emits radiation purely as a function of its temperature. This idealized object came to be known as a blackbody.
In reality, it is challenging to find objects that behave as perfect blackbodies. However, the concept of a blackbody is a useful theoretical construct that allows scientists to understand and analyze the properties of radiation emitted by objects based on their temperature.
The study of blackbody radiation has been crucial in the development of the field of quantum mechanics and our understanding of the relationship between temperature and the electromagnetic radiation emitted by objects.