The History of Blackbody Radiation
As blackbody radiation is a specific case of thermal radiation, the history of blackbody radiation begins with the study of thermal radiation. In 1800, astronomer William Herschel performed an experiment where he created a spectrum of light with a prism and measured the temperature at different parts of the spectrum. He found that the temperature varied throughout the spectrum, which implies that thermal radiation is related to light waves.In addition, he found that the temperature was highest below the red part of the spectrum, where there was no visible light (Elert). These wavelengths with the highest temperatures correspond to infrared light, which, for most bodies on earth, is the primary vessel of thermal radiation.
Assuming a human to be a blackbody, the wavelength with the highest intensity of radiation is around 10^4 nm, which is in the infrared portion of the light spectrum. As shown, objects below the temperature of 3000 K will also have their maximum intensity in the infrared portion.
The next innovator in the research of blackbody radiation was Balfour Stewart, who compared the intensity of radiation of lamp-black surfaces with the intensity of radiation from non-black sources at the same temperature. He found that the lamp-black surfaces had the greatest absorption of radiation, as well as the greatest intensity of radiation from themselves (Wikipedia). The first point is logically apparent due to the way thermal radiation is carried by light waves. Since black surfaces absorb all light, they will absorb the greatest possible amount of thermal radiation.
A German physicist, Gustav Kirchhoff, independently discovered Balfour's findings just a year later, and expanded on them with his theory of thermal emission. His theory states that a surface in thermal equilibrium has an equal capacity of absorption and emission of thermal radiation. Therefore, a body that perfectly absorbs all thermal radiation, a black body, will have the maximum possible emission of thermal energy. In addition, his theory proved that the ratio between emission and absorption is a function of only temperature, and no other variables (Robitaille). The relationship between absorption and emission was known before his theory, but Kirchhoff rigorously proved it and expanded the idea, coining the term "blackbody" for a body that absorbs all thermal radiation. This is, of course, an idealization that does not occur in nature, but many bodies are very close to being black bodies, and can be expressed as such without losing much accuracy.
Max Planck in 1900 found the function that determines the ratio between the emission and absorption of a black body. His formula showed that radiation increases universally for every wavelength as temperature increases, and that the wavelength which corresponds to the highest radiation decreases as temperature increases (Robitaille). Therefore, for very hot objects, the maximum radiation is within the visible section of the light spectrum, and at extremely high temperatures, even in the ultraviolet. His formula can be found in full on the formulae page here.