its high school physics. there are 3 types of heat transfer:

1) conduction - either through solid materials or different things in direct contact. ie intercooler and air. only surface treatments that help the even distribution of heat can help here. black paint doesn't do that and won't help the intercoolers efficiency. the colour of the surface is irrelevant to the conduction of heat. It depends on the thermal conductive properties of the substances involved - ie alloy and air. air is quite a poor thermal conductor, alloys are generally very good.

2) convection - heat transfer through liquids (includes gasses). while convection currents don't come into play in intercoolers, forced convection does. air is forced around the intercoolers internal and external surfaces to transfer heat. forced convection increases the efficiency of air as a thermal conductor. surface colour has no impact on convection or forced convection.

3) radiation - heat energy given off by a body via particles transmitted away from it. This would only be relevant to the operation of an intercooler if the car was sitting still. or if the intercooler was fitted in a box out of direct contact with airflow. ie its not relevant. surface properties that affect heat emissivity (ie matt black vs polished surfaces) effect this type of heat transfer.

if you still don't believe me, ask yourself why a water spray helps intercooler efficiency. clue - it has 23 times the thermal conductivity of air. And the evaporation and forced convection also provide a cooling effect. does nothing for radiation though. but air to air intercoolers don't rely on that method of heat transfer to work. they need airflow for the forced convection principle due to air's poor thermal conductivity.