[IvanK]

Air Publication AP1530B Blenheim MKIV Pilots notes state 2 position variable pitch props only for MKIV
There are only 2 settings Coarse or Fine. No CSU was fitted.

AP1530B Section 2 Handling and Flying notes for the pilot. Introductory notes
(iv) Variable Pitch Airscrews - The airscrews are of the two pitch type i.e. Fine and Coarse. Fine pitch is always used for take off, for climbing on one engine and when approaching to land. Coarse pitch is used for all other conditions of flight, except when throttled well back when fine pitch may give smoother running"

Or purchase the DVD "Flight of the Blenheim"... there is specific discussion on the use of the 2 pitch prop there.

The MKI and MKIV were 2 position variable pitch props.

The equivalent publication for the MKV states CSU props for the MKV.