Actually the props are not fully variable, they are two-speed props. The slider animation is wrong (it should snap between top and bottom position like it does in the Spit Mk.I for example) and this gets many people confused, but if you fiddle with the prop pitch while the engines are still off you can see the blades changing pitch and you'll see it only has two positions:
The bottom of the pitch slider's travel range corresponds to full coarse pitch, everything above that is full fine pitch.
In other words, setting pitch about 50% of the way has no effect whatsoever, it's still running at full fine.
Mixture should be similar, it's only normal (auto rich) and weak (auto lean), but i don't know if it works like that and it's just another slider bug, or if it works like a fully adjustable manual mixture control which would be wrong, since i have no way of knowing/confirming what mixture i'm running.
Also, the flaps should be more like the Hurricane with up, down and center positions for the flap controls, meaning that we should be able to get partial flaps. When the outboard fuel tanks are loaded 15 degrees of flaps is recommended.
The following are subject to how close the in-game CEM and FM is to the real thing:
Cowl flaps should be open on the ground to prevent overheating during taxi, about half-way open during climb and fully closed during cruise/level flight because they still allow enough airflow through at higher speeds to provide sufficient cooling. The Mk.V manual i managed to dig up via google even goes as far as specifying fully closed cowl flaps for take off.
Changing from fine to coarse pitch should be done almost immediately after take off at a speed of 120 mph IAS, while keeping the aircraft level (no climbing) to accelerate as fast as possible and help with engine cooling.
Climbing speed is 150 mph IAS up to 10000ft (subtract 10mph for every 5000ft above that), and single-engine safety speed is 120-140mph (manual says 120, a vintage article i once saw says 140, you can probably climb with some care if you get 140 to run the circuit and land).
This is all straight from the pilot's operating handbook which i sadly can't link here for copyright reasons (wouldn't want the forum admins getting in trouble).
I do however have a thread where i copied some of the Blenheim Mk.V manual parts, i will go and update that one with the corrected information:
http://forum.1cpublishing.eu/showthread.php?t=23192