Hi thanks for this. I have a funny feeling that the problem is a compatability problem. I am using yongnu speedlite yn560. It is not a problem with the live view screen going dark. The live view stays active but the flash just does not fire unless I turn live view off. Perhaps because you are using canon camera and Canon flash all is ok? If anyone is successfully using a 3rd party flash with a Canon 550d utilising live view then please let me know how!. I have since read somewhere that most Canon Cameras you can deactivate "silent mode" to use live view and a 3rd party flash but not with the 550D? I suppose its not a huge problem but is something I wish I'd known before buying a 3rd party flash
