To get the blue key on Doom 64 level 18, Spawned Fear:

(Note: For a video demonstration of acquiring all the keys
on this level, click here.)

First, go into the room where the blue key is and take a
look at the key.  If there are three short vertical bars in
front of the key, blocking access to it, then scroll down
to REMOVING THE BARS, below.  Otherwise:

Just outside the door to the blue-key room is another room.
One wall of this room is lined with what appear to be five
switches in a horizontal row.  The switch closest to the
blue-key door is an impact-activated switch, and it will
temporarily lower the pedestal that the key is sitting on.
(Note: Before activating that switch, it can be useful to
have the door to the blue-key room already open.)

To activate that switch, stand on the raised platform next
to the blue-key door, and shoot the switch with a pistol,
shotgun (either one) or chaingun.
This causes the blue-key pedestal to drop down to floor
level for a few moments, then rise again.  In the meantime
you can grab the key.

(Notes: In the original N64 game, but not in modern retail
ports, that switch can also be activated by attacking it
with fist or chainsaw; And in modern retail ports, but not
in the original N64 game, that switch can also be activated
by shooting it with the Unmaker weapon.)

REMOVING THE BARS
-----------------
If the blue key is blocked by vertical bars, you'll have to
lower them by activating a switch, and/or killing one or
more (varies by Skill level) Barons of Hell.  The Baron(s)
and switch are a few rooms away from the blue-key room.

First, from the blue-key room, walk out the door (into the
room with the 5 switches on the wall), and step through the
next door.  As you step through, turn right and follow the
right-hand wall, passing the first door you come to, then
look for the next door.  It'll have a switch just outside
it, to the left.

Open that door and you'll see a chamber ahead of you, with
another door on the far side, across a square pool of green
slime.  Activate the switch next to you (next to the door
you just opened), and this will open the door on the far
side of the slime pool.

Kill the Baron(s) that you released from the far chamber,
and a floor grate will rise out of the slime pool, allowing
you to cross unharmed.

In the far chamber, you'll see a switch ahead of you.  If
you killed every Baron in that chamber, the vertical bars
in the blue-key room should have lowered, and you can
ignore the switch.  Otherwise, you can activate this switch
to drop the bars that were blocking access to the blue key.