This may not be specific to your question and environment, but Ivan Pepelnjak recently held a great talk at a conference about general virtual firewall concepts:
https://www.youtube.com/watch?v=0ineR4h66Nw
https://www.troopers.de/wp-content/uploads/2012/12/TROOPERS13-Virtual_firewalls-the_Good_the_Bad_and...
Also some nice posts:
http://blog.ioshints.info/2012/11/what-exactly-are-virtual-firewalls.html
http://blog.ioshints.info/2013/04/resiliency-of-vm-nic-firewalls.html
http://blog.ioshints.info/2013/04/are-you-ready-to-change-your-security.html
Perhaps you are thinking too much in the "old", physical way for this, though virtualizing firewalls doesn't mean you have to re-invent the wheel.
But it seems that your future road is set for convergence with your planned blade environment anyways already, so it's either only a matter of time or you should just leave the firewall physical if you're uncomfortable with that.