This has been reported by a number of people and this problem has been reproduced.
Flash player Version 10,0,45,2 (current at time of posting) appears to break the existing Blitz client.
The cause of this problem is unknown right now, but we will work to resolve this over the next few days.
Therefore, to play blitz, DO NOT upgrade your Flash player unless you have good reasons to (for example, a security flaw in Flash needs patching urgently.)
Sorry if you are inconvenienced by this issue, hopefully we can work around whatever problems Adobe have introduced.
A rebuild against Flash 10 player appears to solve the primary issues (the client works again!), but creates other rendering glitches which we will be working to sort out and release today, with luck.
edit : So, once it is fixed, everyone could be required to update their flash player. I'll post more when we know for sure
A new build has been released so the blitz site is back up - but you may now need to upgrade to the latest Flash if you haven't done so already.
Check your Flash version here
