Premonition is not very different from my Architect card in the first post of this thread. It's more powerful, but I think some of the interest of Architect comes from not being able to *completely* architect your next turn, only a good chunk of it. As such, yours might have a little less AP, but I suspect that's a drawback of both of them. I think the idea is interesting enough that it's worth it.
Tariff, as you say, sounds like a card that slows the game down. If you Tariff up the Provinces, you still want to buy them -- you just have to rev up your engine, which may already be weighted down by previously-purchased Provinces. I suppose if you want to go for a three-pile ending, it would be a good move, or if you have an engine that consistently generates more income than your opponent's, so you're overpaying anyway. At first I was thinking this was too close to Embargo to be interesting, but especially that last situation would make it quite different: embargo always affects players equally, but this card might not. If you have $8, you can't buy a tariffed Province at all, while another player who comes up with $9 can. Still, I'd have to try it out in a few games before I could judge whether it's interesting in practice. (Oh, another comment: I have no idea why you priced it at $6 when Embargo is $2. Pretty sure I'd never buy it at $6, but maybe I'm not foreseeing some great use of it.)
Blockade, to me, is the most interesting card of the three. Its effect is temporary, which distinguishes it from Embargo, and it opens up a new strategical consideration. A lot has been written here about the importance of knowing where your own cards are, but how about knowing where your opponent's cards are? If you know, for example, that your opponent's deck is almost gone and he hasn't played his Platinum yet, it might be a really good time to blockade the Colonies. On the other hand, if you suspect your opponent has played out all his good cards this time through the deck, maybe blockade a lower-value Victory card instead, gambling that he won't be able to afford Colonies.
The strategical approach is not that different from playing with Contraband, the difference there being that (barring some weird combo like Masquerading/Swindling a Contraband into an opponent's Venture-heavy deck), an opponent can only make himself vulnerable to Contraband by his own hand, whereas Blockade is an attack you can initiate yourself.
It's possible that, in practice, Blockade won't play as interestingly as it does in theory. Especially since I suspect that it would be a very situational card. (Note: These things are true of Embargo, too.) But I certainly think it's worth experimenting with.
