Some more input;
Mark, agreed with your point 1, black ones go harder, testosterone is probably a key ingredient to increased aggression and strength at least in the areas around Indonesia from my experience. The darker fish are not as common in the Indonesian fishery and I note the black ones generally fight 5-10kg above their weight.
point 2, that is interesting, also consider camouflage, the dark basaltic rocks of hawaii and Eaio appear to produce a disproportionate number of dark fish, perhaps the darker coloration here is a result of natural selection where darker fish are more successful in ambush situations. Or it may be a case where the fish can readily change colour to match the back drop. Interesting to note also are localised dark spots in lighter coloured fish, this appears to be related to areas where the lure has rubbed on the fishes face.
It's an interesting point and I think there may be several reasons for dark fish but I'd presume the case of the inordinately strong + black fish to be males.