ORIGINAL: aspeed
I was always under the impression that a coating traps the heat in alum. at least. I believe a sandblasted finish acts as miniature fins and covering them would slow the escape. Anodizing IMHO is one of these. It is more of a decorative finish. It is easier to wipe clean and looks good. It may be an advantage to keep heat in on a motor, say on a head to keep the plug lit on idle, but I think a natural finish would be more desireable in most cases with a glow motor, and muffler at least. Having said all of that , it probably isn't a measurable difference. I think an car radiator gives off the heat better from the black coating mostly because of the metal powder mixed in with it. It is supposed to be like a metal flake paint. I still like the look of my LA .15 and .40 over the natural finish but use the natural ones for racing just in case. (I never win either way though)
The process I was referring to is not a "coating" like paint. It simply changes the color of the aluminum. Like the "blue" on a gun. Black is supposed to be a better color to radiate heat from........ Like I said, never tested the theory, but I liked the look of the black motor.
I agree a lightly blasted surface give much more surface area to radiate heat from.