The blob is simply the encrypted results of the results of a zone's select() call to get and weight hosts available for a new instance. The decrypted blob is simply a list of dictionaries of form: [{weight: weight, hostname: hostname, capabilities: capabs}]

Take a look at the _scrub_build_plan() method of nova/api/openstack/zones.py to see where the encryption is handled.

More info here on Zones and Distributed Scheduler

http://nova.openstack.org/devref/