In my attempts the only commands related to level bombing from this list:
That work in Blenheim are:
"Previous bombs distributor mode", that chance (drop bombs) to SINGLE and
"Next distributor mode", that change to SALVO.
When you drop the bombs this switch's in bombardier/navigator position
are lowered - click in then make no effect, only show pop up window.
After drop the first two bombs (in SINGLE mode) - notice two switch lowered, related to "Port" bomb bay.
@ knucklebutt
Are you sure that your Blenheim carry bombs?
Sokol1