Yes I agree Heather your research is probably right and it would have been easier just to paint over a clear part! And I'm surprised that with Airfix's current level of research and attention to detail they have made such an error? Just a long shot but are there 2 parts in the kit, 1 clear glazed and 1 frosted for the 2 different night or day fighter schemes?
I just had a closer look.
There are three cockpit glazed parts, but only two (G4 and G11) are intended for this MkIF boxing according to the instructions. Both parts have frosted lower nose panels. Although the paint guide shows the glazing to be clear here for the No 54 OTU version, I think those panels should be painted over for the radar-equipped plane. One of my references shows a MkIF in winter 1939/40 at RAF Wittering, carrying the camouflage and markings found in the box for No 23 Squadron, and that shows a shiny nose panel which intimates unpainted and clear.
Part G12 is an alternate port nose - with clear unfrosted panels, mark you - which I will assume is intended for a future MkIB kit: the difference is the layout of the pilot's side windows. There are bombs in the kit as it stands, which indicates plans for the bomber variant and, and one might hope, a MkIV in due course. Having checked my references, perhaps G12 is intended for the MkIF, and the instructions have shown the wrong part.
This needs a proper Blenheim expert to unpick.