How to acquire the blue key in Unholy Cathedral:

There are 4 large teleporter pads around the perimeter of
the wide courtyard that dominates the center of the map.
Go to that courtyard, and step on the northwest teleporter
(check your Automap to find the correct teleporter).

You'll re-appear at the northeast corner of a small, square
area, and you'll be facing 2 doors at the opposite corner.
Approach the doors and open the right-hand (west) door, and
step through.  Turn right, to face north; then follow the
left-hand wall along the curving hallway until you come to
a booth-like structure in the middle of the hallway.
Walk into and through this booth, and a metallic door will
open nearby, in the north wall.

Go through the door to enter a large area that has 8 short
passages branching off to the east and west.  Walk forward
to the north wall of the room, and then turn left to face
west.  You should be able to see the blue key where it sits
on a pedestal at the back end of the northwestern passage.

Turn back around to face east.  Just to the right of the
opposite passage (towards the northeast corner of the room)
is a slightly raised booth-like structure.  Step up into
that booth and Activate the switch you'll see there.  The
pedestal will drop down, allowing access to the blue key.

PC Doom players can click here to access a demo that shows
how to access both keys on this level.