The chart on page 155 says that Free Actions essentially are so fast that they don't provoke and attack of opportunity. Basically, the Sorcerer doesn't even need to gesture to use Defensive Blast - it just happens with a thought.
The caveat is the usual sub-note 1 for that chart: if you move out of a threatened square while doing any action, even a free action, you will provoke and AOO.
The same page offers the explanation of what is allowed during a Free Action, but it simply says the following, leaving things open to in-game situations and GM sensibilties:
"You can perform one or more free actions while taking another action* normally. However, there are reasonable limits on what you can really do for free."
The asterisk ther is mine, but I see that as being a place where you can aplly judgement as well. If a previous action or an action which that Free Action is combined with provoke AOOs, then the Free Action will as well. My guess is that it will most often be combined in the same round with moving in some way, but remember that because Free Actions take no time for the most part, and many can be used within the span of a single round, you have to look at what else the character is doing to see if an AOO is porovoked.
Alternatively, if you think the circumstances warrant that the blast is "telegraphed"in some way (the sorcerer is acting skittish or particularly wary; the attackers have been suprised by the blast before and are expecting it; he's prone or tied up; etc.) then you can always step in as GM and say it provokes. The description of Free ACtions in general allows for that.
