Black body is a body which absorbs every material and doesn't reflect anything. But in thermal equilibrium the radiation absorbed by i t,has to be emitted to maintain thermal equilibrum. Which means the radiation falling on it is emitted.ie. it's reflecting the radiation doesn't it? When we see at atomic scale the difference between reflection and emission is nothing because a ray is emitted when electron jumps from higher orbital to lower orbital and the incident rays interact with atoms to produce reflected rays. So if the black body reflects every radiation, doesn't the thermal equilibrium will be absorbed.