it's no old wives tale. if you do a search on heat emmisivity you may turn up some university materials explaining that matt black surface are much more efficient at emitting heat energy to air than natural aluminium surfaces. Polished alloy surfaces (as I stupidly did to my rocker cover) are even worse at emmiting heat.

I have no doubt the natural alloy finish became popular for aesthetic value only - what's the point of having the big front mount if no-one can see it?
If you do paint them, use matt black, and mix the paint as thin as possible.

Race engine builders (circuit & rally anyway) have long been painting blocks and rockers matt black inside and out to help with heat transfer. Before anyone mentions drag cars and their intercoolers and polished engines etc - don't bother. Serious drag engines run without any real cooling system at all, just filled blocks. An activity taking under 10seconds is not goint to benefit much from black paint, as there is bugger all time to transfer any worthwhile amount of heat out of the systems anyway. They use the stupidly large intercoolers as much for the heat soak area as for charge cooling!