To get the blue key on level 6, The Crusher:

When you first begin on this level, you'll start in a small
alcove on the east side of a large, darkened room.
Walk through the dark room to the elevator in its southwest
corner, and ride it down to the ground.

The blue key will be somewhere in the huge "crusher room"
beyond the elevator.  The precise location of the key will
vary, depending on the Skill level that you're playing at.

When playing at Skill 1 or 2, the blue key is found on the
crusher platform, located just northwest of the elevator.
Simply walk up the stairs next to the platform to get it.

At Skill 3, the blue key is on the platform located to the
south of the elevator, across a stream of slime.
To get the key there, ride the elevator back up, then turn
to face the platform where the key is.  Step back to put
your back against the bars behind you, then Run to jump off
the elevator and land on the platform where the key is.
(Remember that "Run" in PC Doom games is typically enabled
by holding down the Shift key.)

At Skill 4, the blue key will be located on a platform at
the far west end of the crusher room.
To get the key there, ride the elevator back up, then turn
to face the platform that's just south of the elevator.
Step back to put your back against the bars behind you,
then Run to jump off the elevator and land on the platform.
(Remember that "Run" in PC Doom games is typically enabled
by holding down the Shift key.)
There is a switch here.  When you Activate it, the platform
at the far west end of the room will drop down momentarily,
then rise again.
Activate the switch, and immediately Run to get to that
platform.  Once there, you can pick up the key, and/or ride
the platform back up, and access the dark tunnel beyond.

To download a demo that shows how to access the blue key on
this level, click here.