Blocked as in barrier tape.
The exit looks completely constructed, but it's the only exit at Bang Kapi station not in use.
It is NOT an "exit connecting to the Mall" - nothing there connects to the Mall, it's all footbridges and construction and parking lots and dust, a quite unpleasant walk away from the Mall.
It is just the exit that would be the nearest to the Mall.
Fortunately, halfway between the Mall and the BTS there is a two-storey 7/11, so on your trek through the heat you can get a rest and some refreshments :)