To get the blue key in PSX Doom level 58, The Mansion:

The blue key rests on top of a large square pedestal.  The
pedestal is in a room located next to where you first
appear on this map.  There are two ways to get this key:

1) Strafe-run off of the north ledge in that room to land
on the blue key.  Using a simultaneous combination of
Run-forward plus side-Strafe, you'll move slightly faster
than plain running (allowing you to jump a little farther),
but you'll move at a weird angle.  You may have to attempt
the jump a few times before you make it.

2) In the middle of the south ledge in that room there's a
door.  Go through it, then open the door in the wall to
your left.
You'll be looking into a room where four tall columns are
rising and falling.  On top of each moving column there is
a teleporter pad.
When the teleporter nearest to you (closest to the door you
just opened) drops all the way down, Run and jump onto it.
You'll materialize at one end of a short, closed hallway.
This hallway is located above and to the north of the blue
key.  Run south and the hallway's south end will open up,
and continue running to jump the gap from the north ledge
to the blue key.