IIRC, the very first "off-tools" TR2s (TS1LO and TS2) had black engine bays. But both of these were virtually hand-built production cars destined for Auto Shows (Canada, Ireland, respectively). I don't know offhand how quickly Standard-Triumph went to body-colored inner fenders, etc. Lou? anyone else?
Otherwise, I agree with all of the above. It's a: a "look"; b: expedient when "freshening" a car or just easier to maintain; and/or c: covering up something. :wink: