Hi all, first post so be kind.. I suspect the Lancaster cockpit colour changed to black due to reflection on the inside of the canopy thus reducing the pilots view outside. I suggest this as I worked for a few years in the offshore rotary world and the pilots immersion suits and life jackets were all dark blue for this very reason.
I reckon the change came about from aircrew feedback on ops and only became a problem when the sky became illuminated with searchlights and flares etc. Might suggest why the bomb aimers position was the first to be painted black.