I have been in a similar situation during a tube strike in London some years ago.
Due to the strike Blackhorse Road station was closed, and London Overground services were not meant to stop.
However the service I was on, going towards Barking, did stop, but when we went upstairs the gate leading into the London Underground part of the station, and thus the exit, was locked.
Therefore I, and several other people, were trapped on the London Overground platforms with no legal means of escape.
Fortunately another train back towards South Tottenham was due, and it stopped to pick everyone up, and those who wanted to alight at Blackhorse Road were warned not to.
Had there been no means of escaping by train, I would have dialled 999.
Needless to say, my complaint to London Underground was met with the standard reply of "..nothing to do with us Guv, it's London Overground's fault..", and nothing ever came of the incident.
The incident at Blackburn needs to be investigated thoroughly as it is surely a serious breach of Health & Safety legislation to leave passengers locked inside a station with no means of escape. What if there had been a medical emergency or a fire? (Although presumably in these circumstances the fire brigade would have broken down the door or smashed a window.