To get the blue key in PC/Mac TNT level 9, Stronghold:

The blue keycard sits on top of a crate, in a room that is
located near the middle of the south edge of the map.
Just behind that crate, along the east wall of the room, is
a row of barrels.  Destroy these barrels.

A few paces to the west of that crate, in the south wall of
the room, is a doorway.  Step into that doorway and turn
left.  Proceed forward (east) through a series of metallic
doors, then follow the stairs up and around to a wide, open
window.

This window looks out over the room you were in a moment
ago, and the crate where the blue key sits is close by.
From here you can simply jump (Run across the gap) to the
blue key.

(If you were at this point before but were unable to jump
across, it was because the barrels on the floor below were
blocking your path.  In the original Doom/Doom II engines,
players cannot cross over or above barrels at any height.)

PC TNT players can click here to access a demo that shows
how to acquire the blue key (and red key) on this level.