The evidence is drawn from a selection of sources that refer back to the evidence we started with- Bob Kane telling us that he thought Bill Finger's design was meant to be black and grey.
I don't find it compelling, not least because Bob Kane's opinions in the 1940s are irrelevant today. Some more modern artists might accept your "extended metaphor", but it clearly breaks down where you have story in which Batman wears black and grey, with a flashback to a former time where he was wearing blue and grey.
I don't doubt that Batman wears black and grey at times, and did so in his earliest appearances. But designs evolve, and I won't be convinced that in this picture, for instance...
...Batman is supposed to be wearing black, while Alfred is wearing- what?
There is an inconsistency underneath all of this, certainly, but I don't find that problematic. Batman used to wear black and grey, now sometimes wears blue. If I had to impose a logical sequence on the matter, then I would submit that the "new look" costume of 1964 onwards introduced the yellow oval and made blue mean blue.