An uncle of mine had a Lanchester. It was a somewhat lighter shade than the standard RAF blue but could be mistaken for it. The car was black above the bottom line of the windows with black mudguards. I've never seen any reference to Lanchesters being used as official RAF cars - these ( at least the larger saloons ) seem to have been Humber Snipes and Standard 10's, as 4 seaters, with Humber Pullmans as 6 seaters. It's possible, of course that someone acquired it and had it painted RAF blue but, bear in mind, that the blue finish was replaced from August 1941 by a khaki green with some overpainting with tarmac green. Even so cars issued to officers of air rank ( Air Commodores and above ) were not subject to the camouflage scheme.
My guess is that the blue paint is it's original private owner finish.