I'm not convinced that no WW2 motorcycle was blue.
I saw a speedo recently in original blue (RAF I think) with a plate on the reverse marked "1942" which seemed to point to some motorcycles being that colour, Nathan (remember him) had a NOS MT1130 that was blue? and I have a fag end tail lamp lens in blue which couldn't have existed before or after the war, even if this war for a truck or car it proves a point.
I believe that Ariel probably did supply some machines for the RAF and Admiralty in non standard Army colours.