Having spoken to Alan in Technical Support today, he confirmed that there is an issue in vCenter 5.1 whereby VM console screens remain black for too long, in my case it is around 30 seconds. Prior to 5.1, this would only take 2-3 seconds.
PLEASE fix this issue. I understand that the native Vsphere client will be phased out at some point in the future but until then, this issue really needs to be addressed. It is a real pain!