Initially, the Air Ministry favoured blending, but found that, at times, the blending was carried out by simply pulling the spraygun away from the surface, which led to almost-dry paint hitting the surface, giving a rough surface, and inducing drag. In early pre-Dunkirk 1940, following a meeting, at which a professor said that blending was a waste of time, a general order, on external finish, was issued, in which it was said that mats could now be used; oddly, there’s a 1937 Spitfire camouflage pattern drawing, in the National Archives, which carries exactly the same reference to the mats as appears on the later Mk.21 drawing, supplied by Tony (above,) which gives a hint that Supermarine never used blending, for some unknown reason.
When the war was in progress, Spitfires were being repaired at M.U.s, and factories of the Civilian Repair Organisation, who might not have had access to the mats, so would probably have had to revert to blending; one man I spoke to said that he “free-handed” an Oxford, but the foreman made sure that his overspray did not exceed half an inch.
In August 1942, Supermarine (and others) went from cellulose to a synthetic finish, which was smooth and matt (something paint manufacturers had found difficult, even impossible, to achieve in cellulose,) so maybe blending was less of a problem, but I’ve found no evidence of that, so far. Around that time, a new trade, of Aircraft Finisher, was created, which would/should have ensured better surfaces.
Post-war, blending returned, but, with a gloss varnish on top, maybe drag was less of a problem.