A blackbody is a conceptual solid mass that absorbs all electromagnetic radiation incident upon it, at all angles and all wavelengths. A blackbody does not reflect nor transmit electromagnetic energy.
A blackbody also-diffusely-radiates all the electromagnetic energy it absorbs.
Because a blackbody absorbs and emits all radiation it encounters, it is considered in thermal equilibrium with its environment…again, a conceptual configuration.
This video (14:17) explores the concept of the Blackbody Radiator in thermography.