Here's another source of heat. The idea is that the combustion reaction of the burning logs heats up the metal case. The case is painted black because it then emits infra-red heat radiation into the room. I prefer radiant heat to convection. It travels at the speed of light (it is infra-red light) and so it instantly feels hot. It takes longer for my convection heater to circulate thermal energy by means of density changes in the air. But note that although it true that matt black surfaces absorb heat radiation well, the opposite is going on here. A matt black surface is emitting heat radiation well.
