Which cards are you thinking of that mess with opponents but don't count as attacks? When I think of pseudo attacks, Possession, Masquerade and Tribute come to mind. For those 3 cards, you can't selectively screw over your opponents, there just happen to be certain deck styles which those cards can exploit (though I'll admit Possession is absolutely infuriating). Blockade on the other hand lets you inspect a player's top card, and either leave it there or discard it. This is clearly the attack part of Spy, so I'm not sure why that doesn't count as an attack.
I totally overlooked the existence of Spy when I made this. I was focused so much on the shuffle condition. That doesn't mean this ability shouldn't be this way, but it got me thinking. Here's the full card as I alluded to with the extra "attack" ability:
Blockade #2 (4)
Action - Attack - Duration
+2 Cards
At the start of your next turn: +1 Card
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While this is in play, whenever a player shuffles his deck, look at the top card of it afterwards. You may have him discard that card. Players may reveal Reaction cards as though you just played this.
You'd need something like that to make Reactions work to the actual attack. It just seems better to say this isn't an attack and let thing's slide, like with Ill-Gotten Gains.
can I do a Spy attack per Blockade in play whenever an opponent reshuffles?
Yes, each of them triggers and you'll spy one top card at a time.
Uh, what happens if multiple players have one in play? Heck, what happens if one player has several in play?
See above for one player having multiples. They resolve separately, so you'd dig through one card at a time (or just leave the card there eventually). As for multiple players, there isn't current precedent for how it would work (that I know of). So, it's time to make one: should it resolve in turn order or reverse turn order? I'm used to Magic, where it goes it reverse. That is, if you and I both have Blockades out, and I shuffle my library on my turn, my Blockade would trigger, then yours would trigger. After all triggers occur, yours resolves, then mine resolves (Last In, First Out). That doesn't mean it needs to work the same here. Dominion isn't concerned with these type of timing issues. I'd say just resolve them in turn order. Thus, if I shuffle and we both have one out, I Spy, then you Spy.