A perfect black body has the unique characteristic feature as

A perfect black body is an idealized physical object defined by its ability to absorb all incident electromagnetic radiation, regardless of frequency or angle of incidence [t1][t2]. According to Kirchhoff's law of thermal radiation, for any body in thermodynamic equilibrium, the emissivity is equal to the absorptivity [t3][t5]. Since a perfect black body has an absorptivity of 1 (meaning it is a perfect absorber), it must also have an emissivity of 1, making it a perfect radiator (or emitter) for its given temperature [t4][t6]. It emits the maximum possible thermal radiation compared to any other object at the same temperature [t8]. Therefore, the unique characteristic of a perfect black body is that it acts as both a good absorber and a good radiator [t7][t9]. It does not reflect or transmit any radiation, appearing black at room temperature [t3][t8].

Sources

1. [1] https://acd-ext.gsfc.nasa.gov/anonftp/acd/daac_ozone/Lecture4/Text/Semifinal/blackbodyintro.html
2. [2] https://www.sciencedirect.com/topics/computer-science/blackbody-radiator
3. [3] https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation