For blocks, it's tricky:
A zone border might be defensive if you're trying to prevent attackers from getting in to your zone.
A zone border might be offensive if you're trying to prevent an enemy from escaping.
Putting a block on someone to hinder their attacks might be offensive magic
Putting a block on yourself to stop incoming attacks would probably be defensive.
Overall I'd say:
If it targets an enemy, and it's a baneful effect it's usually(probably) offensive.
If it targets yourself and it's protective, it's probably defensive. (some buffs might be offensive, though).
As with Scene aspects...I'm not sure they're either. Anyone can invoke them. I could use the fog to hide (defensive) or I could use the fog to sneak up on enemies and ambush them (offensive)...of course, your enemies can do the same. Overall, it feels more defensive to me.
I'm not sure if 'intent' comes in to play with focus items...magic is such a personal thing, after all...
My wizards always have a zone border rote. It's specifically designed to keep enemies at bay so I've assigned defensive foci to it. On the other hand, if my primary intent was to keep enemies from fleeing, I might have assigned offensive foci. Of course, zone borders affect everyone equally, friends and foes, travelling in either direction...so it's kind of neutral in that way.
Edit: You know, there may be certain spells where no foci apply. And I see nothing wrong with that.