How to get the blue key in Doom level 21, Mt. Erebus:

Along the middle of the north edge of the map is a large,
diagonally-oriented building, with a small wooden door in
its southeast wall.
At low skill levels, the blue key will be just outside of
this door.

To get the blue key at higher Skills, open that door, step
in and turn left.  Scan the middle of the southwest wall,
and look for a small candle on the floor at the base of the
wall.  Open the wall panel just behind the candle, and step
on the teleporter that's revealed.

You'll re-appear in a smaller, separate area of the map,
located just east of the rest of the level, and you'll be
standing on a narrow platform that surrounds a small square
building.

Go to the east side of this building, and open the small
door there.  Step in and Activate the switch at the
opposite side of the room.  This opens up the southeast
corner of the room, where the yellow key is; Grab it.
Activating the other switch in the north half of the room
will open the northeast corner, allowing access to a
radiation suit.

Leave this building, and open the yellow-keyed door at the
north end of this area to reveal a teleporter.  Step on,
and you'll re-appear at the northeast tip of a 3-spoked
building in the northwest corner of the map.

Turn around and go forward and left, to the southeast spoke
of this building, where you can find the blue key.
Go to the west-side spoke and activate the switch there,
and the tip of the northeast spoke will open up, allowing
you to leave this building.