Actually, you want a polished surface on the bottom of the manifold, so it doesn't absorb the heat from the oil, and a black surface on the top so it emits the absorbed heat from the oil and water to the surrounding air.
On the last post, David Pozzi made a good point about Vizard's book stating that the manifold's top surface should be polished in order to eliminate the cool intake air from being heated by the surrounding environment. This is true for a drag car. In a most drag applications, the engine really has no heat built in it, so the water and oil have not had a chance to heat the manifold completely, and there is enough volume of fuel/air at WOT to cool the intake significantly. However, in a street application, the water and oil are very hot, and the manifold has only part-throttle fuel/air moving through it. In the case of an aluminum manifold, the heat created by the water and oil needs to be dissipated into the atmosphere as quickly as possible to keep manifold temps down. Therefore, you need a surface on the top of the manifold which will dissipate heat quickly. A polished manifold WILL NOT do this, so painting the manifold black will help since black emits heat faster than other colors. In fact, in a street application, cooling fins on the top of the manifold, much like a heat sink in an amplifier, would help dissipate this heat (but it wouldn't look good).
The bottom line to manifold coatings/colors is you have to take into account the temperature gradient of the manifold and surrounding air. If the manifold temp is much higher, you need to dissipate the heat into the air, so a black color will accomplish this. In a race environment, where the manifold could be cooler than the surrounding air, due to the evaporation of large amounts of fuel, the manifold could be thermally coated to eliminate any heat transfer into the surrounding air. The ideal situation in a race engine would be to thermally coat the inside of the intake ports and the bottom of the manifold.
I know this is off the subject a bit, but I don't think painting aluminum heads will hinder the heat transfer enough to cause a concern. I know the Edelbrock heads on my '69 will be painted, because I like the sleeper look, too. Of all the aluminum heads on the market, I think the Edelbrock heads are the hardest to discern from stock heads when painted. They don't have a lot of machined logos/surfaces on them, and look very close to stock when painted. Take care.
69 SSRS Frame-off Resto
81 Z-28 377ci Drag Car