In the Flow General Settings you simply need to check radiation, then insert "radiative surfaces" and select the specific materials to apply to each body and Flow does the rest. It doesn't automatically apply the surface condition along with the solid material definition.
I guess you are working with Simulation Professional, right?
You have to define a seperate radiation surface with epsilon of 0,95 for the concrete block.
If you have an open system you have to define the environmental temperature. This means the temperature in your oven (not the temperature of the layer next to the surface).
It would be easier to do this kind of analyzis with Flow Simulation.