Searching the web for the maximum speed of the Bristol Blenheim I and IV I get a lot of differing figures, upto 285mph for the the Mk I and upto 295mph for the Mk IV. What were the top speeds of the 2 types?
The Blenheim Mk 1, with standard engines, had a 'true' top speed of 274 mph @ 13,000ft. This was using Rotol (standard) propellers (2750rpm +5psi).
With careful cleaning, smoothing (including wing fillets) and engine boosted from +5psi to +9psi, speed increased to 296mph (true).
The Blenheim Mk IV, demonstrated a maximum speed of 255 mph @ 2,000ft. Flight testing also revealed different speeds depending on the manufacturer. A Blenheim Mk IV produced by Avro, for example proved to be 5 mph faster than the original Bristol-made aircraft.
The report goes on to mention that better speeds were expected utilising a special paint, called Camotint.
You really complicated this question ,Greame.
But it seems that these speeds of Blenheim MK.I and IV are almost similar.As you have mentioned above, these depended on many conditions (altitude kinds of propelers,paints,bombloads,etc....