Bespr.Nr. 176 v. 12.10.1942 notes:

Approval of the BMW 801 D at full power. Based on experience to date in its present form, the approval of the BMW 801 D at full power without “Blenden” can be answered for under the following stated conditions:

Chrome-plated valves, Bosch spark plugs, fixed spark plug sockets preferably latest rolled version, nitrated blades in supercharger, strengthened starter shafts. Subject to a reexamination of the empirical data the following procedure is agreed upon.

For deployed engines: Clearance of the non-derated power, “Blenden” remain fitted, examination of the engine for fixed spark plug sockets, equipped with chromed valves, Bosch spark plugs, inspection of the injection timing points.

An instruction is to be published making sure that only chrome plated valves are installed with cylinder change.

For new deliveries and during complete overhaul and repair: clearance of the non-derated power, no “Blenden”, fixed spark plug sockets examined, chrome plated valves, Bosch spark plugs, nitrated supercharger blades, strengthened starter shafts.

There is uncertainty as to the best translation of Blenden in an engine context. Presumably it is an orifice, screen or restricted opening/fitting on the BMW 801 D engine. Another possibility is that Blenden were installed inside the cowling to change the cooling airflow, which might explain the difference between field-modified and new-production aircraft.