It may be true, but the difference would be minimal. Convection is going to remove far more waste heat than radiant losses, by orders of magnitude.
Radiant heat losses increase as a function of temperature differential, and so significant heat shedding won't take place until we'll beyond the operating limits of many engine designs. That's why every engine I can think of employs convective cooling via water or forced air.
It's also worth noting that radiant heat exchange would require any heat shed from the engine to be absorbed by an adjacent or nearby material, typically in direct line of sight. The heat then would need to be pulled from that panel or part in order to continue absorbing heat from the engine..
In other words: a proper mix of coolant will do more for you than a black can of krylon.
