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.