I would do it by making the "Blighted Gold Mine Ability" ability have two levels - level 1 is the standard gold mine ability, while level 2 has space for 6 acolytes.
Then make an upgrade. This upgrade does nothing and is used by no one. Then make 2 triggers: One fires when unit "finishes research" - then pick all blighted gold mines of that player and set the level of their "Blighted Gold Mine Ability" to level 2.
Second trigger fires whenever a Blighted Gold Mine has been constructed (dunno which event gets that exactly) and check if the research for the player owning that gold mine has been finished. If yes, set the level of the gold mine ability to level 2.
I can't guarantee it will work, but it's the only idea I got.