To get the blue key in Plutonia level The Death Domain:

Take a look at your Automap.  Dominating the center of the
level is a huge open courtyard.  To the south-southwest of
the courtyard is a smaller outdoor area, raised up above
ground level, that overlooks the courtyard.

Get up into this raised area.  You can either get to it
through the yellow door in the southwest corner of the
courtyard, or through a convoluted path that runs clockwise
along most of the outer edge of the level.

Once you're in this southern raised area overlooking the
main courtyard, destroy the barrels you see there.  Now
notice the fence that runs along the north edge of this
raised area.  In the middle of the fence is a section that
drops down, and allows you to Run and jump into the trench
in the courtyard where the blue skull key is.

To get this section of fence to drop down, look for a
booth, or short dead-end passage in the southeast corner of
the raised area.  At the back of this booth is a switch.
Activate the switch, then quickly Run for the fence.  The
section of fence will only stay down for a second, so you
have to move fast.

Warning: When you first step into the booth, you may
trigger a trap; a bunch of enemies may start teleporting
into the courtyard trenches.  You may want to go clean them
out first before attempting the jump to the blue key.

PC Plutonia players can access a demo that shows how to get
the keys on this level, by clicking here.