How to get the blue key on the Doom II map Industrial Zone:

(Note: For a video demonstration of the walkthrough below,
click here.)

To get the blue key you'll first need the yellow key, then:

Look for the tower at the southwest corner of the map.
Enter through the yellow-keyed door.  There's a radiation
suit in a little alcove on your right as you walk in.
In the floor is a crusted-lava stream.  Follow this stream
south, into a darkened tunnel.
Follow this spiral tunnel clockwise and around to its far
end, and Activate the switch you'll find there.
Go back out of the tunnel, and just as you emerge, look to
your right for a stairway.
Follow the winding path of stairs all the way up as far as
they go.
You'll come to a face-like switch on your right.  Activate
it to lower a small lift.  Get on and ride up.
Turn right and head east, around the top of the rampart,
until you get to another face-like switch.  Activate it.
Now go back down to the inside of the base of this tower.
Just to the southwest of where that radiation suit was you
should now see an indentation in the wall.  Open this panel
to reveal the blue key.

PC Doom II players can access a demo of this level showing
how to acquire the blue key, by clicking here.