No, it isn't. You are in fact being extremely subjective in forming your opinion on the narrowest range of "evidence". You have a quote from Bob Kane, in which he says that he considered Batman to be wearing black in his early appearances, and you have found other sources who allude to his comment and consider the matter no further. Against that, you have to balance at least sixty years of artistic development where blue has quite clearly been used to represent...blue. Your "extended metaphor" doesn't stand up to scrutiny when a blue-clad Batman is stood next to a character wearing black; nor when a black-clad Batman remembers his past, when he wore blue.