I asked the same question a while back with the earlier ultimate blackness and got a similar response.
what do you mean warm?
In reference to warm: I think the poster means is it warm since it is capable of absorbing virtually all of the impinging energy.
The principle of conservation of energy. When a black body absorbs light the energy in the photons does not cease to exist. Loosely speaking, it is converted to random, disorganized mechanical energy, in other words, heat. Think of a black car on a hot day.
The heat is converted back to electromagnetic energy according to Stephan-Boltzman’s law.
https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant
Sunlight warms the surface of the earth (which does not absorb all wavelengths, much sunlight is reflected back into space), and air cools the surface of the earth, or rather, the surface of the earth warms the air.
The mechanisms that allow heat in the atmosphere to be radiated back into space are at the heart of the global warming debate.