Blocks do not affect defensive rolls.
Nor do they place an aspect.
... unless done as the 'freebie' action that maintaining a grapple allows you to perform.
Ok, I think I understand better what the original question was than I did before. Say Jimmy is walking down the street when suddenly two thugs jump out of an alley. One of them grabs him and puts him into a full Nelson, leaving the other free to pummel the ever-lovin' *bleep* out of him. "In real life", this grapple would be preventing Jimmy from putting up much of a defense; his only hope is to break out of the grapple if he hopes to defend himself.
In DFRPG, grapples don't restrict defense at all, which makes it difficult to represent this sort of situation. The closest you can get is for the thug doing the grapple to use his free bonus action to put maneuvers on Jimmy -- such as "Got you pinned" and "Helpless to defend yourself". The the punching thug can either throw attacks each exchange, tagging the most recent aspect for a +2 (thus in effect counter 2 points of Jimmy's defense) or by piling on maneuvers of his own, then letting loose with a 'one-shot-takedown' attack tagging whatever maneuvers have built up. Which might actually do a decent job of dealing with the situation.
Of course, that's the mundane version; the OP was using a spell to do the grappling. Which brings up a question: does the spell, once in place, get that freebie action each exchange? Or does it get the action, but only if the spellcaster 'concentrates' on it (in other words, it's not really a free action)?
