While pulling the few hairs remaining on my head trying to place blueprints, luckily I discovered this thread (thanks to @Ruxandra for raising the topic and to @red51 for the answer)!
Now, I notice:
1) To "freeze" a blueprint you are placing, [Left Click] is used, while to freeze construction elements [Right Ctrl] is used.
2) To raise / lower a (frozen) blueprint, [NumPad +] / [NumPad -] are used, while with (frozen) construction elements [PgUp] / [PgDn] are used
Also, with the terrain tool:
3) in Mode 1 (Add/Remove), [Left Click] adds and [Right Click] removes, while in Mode 2 (Smoothing), [Left Click] smooths by removing and [Right Click] smooths by adding.
Is there a reason behind this discrepancies?
(Aside: Some of the short-cuts listed above can be customized, other cannot. Even assuming in the future all short-cuts will be customizable, a consistent default set of short-cuts is important for the user experience and to ease the learning curve)