That, for a short while, it was the best at night tells one more about the weakness of nightfighters at the time before airborne radar and in the earliest days with the first primitive sets when the Defiant got them and moved from ground controlled radar backed up by four peering eyes to a short range radar on board.
My problem with the Defiant is that the time lines and claims to how effective it was don't seem to line up.
it performed better than the available alternatives until the radar equipped Blenheims came into full service and were still useful until both were overtaken by the Beaufighter.
I don't know when/if the Radar Equipped Blenheim's came into
full service for example. We know they were using
some Blenheims with radar in the summer of 1940.
According to one source(book) in Sept of 1940 the RAF had, for all practical purposes, 6 squadrons of Blenheims and 3 of Defiants for night fighting. They were trying to put up Hurricanes and Spitfires at night at times. 1/3 of the Blenheims had AI III and there were some AI II installations. Defiants were one year away from getting radar at all.
As noted by others, without improvements in ground radar, IFF and control airborne radar itself wasn't going to be very effective. The 4th thing was VHF radio instead of HF radio which allowed for vastly better communications between the pilots/planes and ground control.
Beaufighters start showing up in squadrons in Sept, in 2s and 3s in Blenheim squadrons. They either have or are soon fitted with AI IV radar. Radar production doesn't always keep pace with airframe construction.
The 2nd radar "kill" (Nov 7th, 1940) was by a Blenheim from a FIU squadron that had been reequipped with a MK IV set. It was done by Flying Officer G. Ashfield who had also scored the first radar 'kill' back in July. 1940 was pretty much just getting all the pieces on the board.
By the end of Dec 1940 they had built about 100 Beaufighters and most of the original Blenheim squadrons had converted. A few squadrons had converted from other aircraft to used Blenheims. Some Squadrons that flew Blenheims over the winter of 1940/41 were converted from Hurricanes.
There were 3 forms of "night fighting" going on in the RAF at this time. This confuses some of the kill totals. There were Hurricanes and Spitfires (and some others) flying around at night and perhaps getting instruction from the ground but target acquisition was pretty much luck, moon beams and enemy plane flying over searchlights and fires on the ground.
The 2nd is night fighting as we think of it. With radar equipped (or not) fighters being controlled by the ground and "patrolling"(flying patterns) certain areas. Final acquisition of target was by the same visual methods.
The 3rd, which was rather effective in terms of kills was operating intruders. Blenheims, Havocs and Beaufighters Flew to France and orbited German bomber bases waiting for returning German bombers to switch on navigation/landing lights or be silhouetted/illuminated by landing flares/illumination. Radar equipped night fighters were rarely (never?) used for this in 1940-41 to avoid possible loss of radar equipment.
The Defiant's 'superiority' over the Blenhiem seems to be this somewhat twist path of the Blenheim going out of night fighter service by Dec/Jan while Defiant squadrons were increasing in number. The Ground controllers got better over the winter, getting more planes, off whatever type, in closer proximity to the targets. The Defiants did score kills in March, April and May but by that time the Blenheims were out of service (a few hangers on?) with better weather and shorter nights as mentioned earlier.