To get the blue key on level 2, The Terraformer:

(Note: For a video demonstration of the walkthrough below,
click here.)

When you first find the blue key, it will be resting on a
short pedestal in the center of the room.  On the back of
the pedestal is a switch.  Activate it, and the pedestal
drops.  When you try to grab the blue key, it disappears.

The blue key will then pop in and out of existence along
the tops of the other pedestals in the corners of the room.
Eventually the key comes to rest on one of them (the final
pedestal is chosen at random).  When this happens, 4 doors
open up along the back of the room, and behind each of
these doors is a switch.  To either side of 3 of these 4
switches are short vertical slabs rising out of the floor.
Go to the one switch where those slabs are missing.

When you activate this switch, the pedestal where the blue
key now rests will drop.  Run (remember your Run button) to
the pedestal before it rises again.

(Note: For a video demonstration of various movement speeds
in Doom 64, click here.)

If the pedestal rises up before you grab the blue key, just
go back to the switch and try again.

If you activate any of the 3 switches that have those slabs
to either side, a section of the ceiling overhead will drop
down briefly and crush you, possibly killing you if your
health is low.