I think you'll find that painting it black would decrease performance. At the sort of temperatures we tend to deal with radiation is a minimal factor. Nearly all your cooling would be via convection (either natural or forced air) and thus bare metal is better than paint due to having one less interface. The only reason you see black heatsinks is because they're anodising them anyway so colouring it black or not makes little difference to convection and can help radiation marginally.
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