u could have a medic blind the trap in the begining of the game, there is a script somewhere for a medic to blind units, I dont know exactly WHAT the trigger is, but u could do that. As for preplaced blind, I donno. Also, yes, the medic can blind traps.

ADDITION:
Also, if you have Starforge, I heard of this little/big project where they tried to find out what placing numbers in the unit's property unknown slots, if you can find that forum, it may help. I asked a question if triggers could detect blindess, they said no...