The media blackout was most likely triggered by a 'DA-notice', however many sites have picked upon this in the last few days:
https://web.archive.org/web/20120721020230/http://newsallianceuk.wordpress.com/2012/03/18/armed-police-and-media-blackout-of-nhs-protest-shows-desperation-of-condem-regime

I think there are 5 types of DA notices. But essentially this would explain the 'mysterious' media blackout. It was probably newsworthy of, at the very least, a small story perhaps not on the front page - but it was literally not reported in ANY mainstream media outlet.

This is just like the 1930s Tories..always evil and the enemy of the working man.