I feel like this is too close to just being a Shock, not a bad hexproof granter. What about "Target creature gains hexproof until end of turn. At the beginning of the next end step, ~ deals 2 damage to it."? Of course, granting hexproof is a green and blue ability anyway.

I think the ability you suggested sounds more problematic in that remembering to deal the damage at the end of the turn rather than remembering the creature has hexproof for the remainder of the turn. The idea came for a twist on simple red instant that dealt damage, and into my mind came the idea for something that could cause a problem for the caster when used on an opponent's creature (in this case it would be if they buffed the creature) so that it couldn't be further burned. It also gains the benefit then of using it on one of your creatures, presumably something big that it'll take damage to gain hexproof, which I think is red, or at least I can see the classic MtG goblins doing that.

I love the concept of this :) But yeah, it wouldn't normally be printed like this, I'm not sure if it's ok or not.

Red hexproof? :(

Well.. sorta. It's gonna be very hard to use this to hexproof your own creature. Usually it's be "Ok, so.. your creature is damaged and about to gain hexproof? I finish it off!"

It's more "Shock with potential downside"

Also a "device" ought to be an artifact, or a creature really; not an instant. And now, trying to find a way to verb this, I come up with "Goblin Cloaking" followed by "Goblin Cloak" - which should ALSO grant damage and hexproof, and we have the wonderful image of a haplass adventurer hiding under a pile of goblins; all gleefully piling on.

I don't know where I was going with this comment, really. But yeah; this is taking away one of blue's things. The Goblin Balloon Brigade already went there, though.