Why use a black body radiator as a standard, when no such thing exists?
It turns out that black body radiation provides us with a set of very precise working equations that relate the temperature of an object to the light it emits. Working from the ideal and using Planck”s law, we can predict the energy distribution across the spectrum for a given temperature. The total emitted power is calculated using the Stefan-Boltzmann law. The wavelength of the peak emission, and hence the color that dominates for this temperature, is provided by Wien’s displacement law. Knowing the ideal case allows us to predict or calculate actual values by correcting for the imperfections of actual hot objects.
h/t to ewmon