How to get the blue key in PC TNT level 5, Hanger:

The blue key sits on a platform.  The side of the platform
that faces into the room is a blank gray rectangle.  Walk
up to that blank gray rectangle and Activate it like a
switch.  You'll hear a machinery noise; that is the sound
of a low pedestal dropping into the floor behind you.

Quickly Run backwards away from the blue key until you step
on that lowered pedestal.  When you step on it the blue-key
platform will drop.  Quickly Run forward to grab the key.

PC TNT players can click here to access a demo that shows
how to get the blue key on this level.