I have spoken extensively with the head engineer at Calsonic - one of the largest cooling suppliers in Japan.
They have done tests between black body radiators and naked alu radiators, and there is, for all intensive purposes FU(K ALL difference in efficiency between the two.
He mentioned they are often asked by cleints to dust the radiators / coolers black for aesthetics only.
I'm sure that still won't put an end to these discussions, but you may be best looking for supposed 'inefficiencies' elsewhere like cooler pipe length and heatsoak there.