Hey Biff,
Surely the point in WW2 is getting out of visual range, regardless of whether you're flying at night or in daytime? Or at least getting far enough away so that, coupled with acceleration/speed, the enemy fighter can't get to you?
I'd be absolutely astounded if Blenheim navigators weren't trained during the OTU phase in determining whether or not a gaggle of EA constituted a threat. As you point out, failure to train to that would result in even worse loss rates. By the time the Mossie entered service, the RAF had 3 years of operational experience in Blenheims and I just don't think it's realistic to assume the navigators were incapable of accurately reporting threats to the pilot.
Again, I'm not suggesting it's smart for a Mossie to tangle with a SE fighter. However, given the operational areas, particularly for the anti-shipping strikes, and given the operational experience of the RAF at the time, I find it really hard to believe that the navs weren't capable of working with the pilot to spot and report threats.
Going back to the F-15E for a second, I was at RAF Honington when the first Strike Eagles arrived just up the road at Lakenheath. At that time, the UK's low flying areas were something of a wild west. Any military aircraft flying in the LFAs was deemed a suitable target to be bounced by any other military aircraft in the area. This was not pre-coordinated but simply arranged on the fly with the appropriate ATC. After the F-15Es arrived, several RAF squadrons attempted to bounce them only to be told "knock it off". Shortly after, we had a liaison visit from some Strike Eagle crews. When asked why they refused to engage with the bounce, they responded that they were still trying to figure out how to fight the aircraft...apparently the MFDs had so many levels and configurations that the WSOs were getting "lost" in the software...and doing that at 250ft altitude was a BAAAAD thing! Happy days....
Cheers,
B-N