See Challenge # 088.

My logic for this challenge is: what's the difference between a two-colour deck and a two-colour deck with a third colour splash?

The two colour deck doesn't play a small number of off-colour producing lands.

That means mechanics that scale with the number of G&U you have aren't really sufficient: it stops a LARGE splash but it's often still worth splashing another colour for removal even if it makes your X/X creature slightly smaller.

It has to be a mechanic that makes having only one or two off-colour lands a significant drawback. But not so significant that you can't compromise: you could have a 2/2 creature with a bonus if "all your lands can produce G or U", but that removes a lot of potential for interesting decision.

So I tried to think of something that would scale, where having 9 on-colour lands instead of 8 still matters, but having 8 doesn't completely kill you.

One answer is hybrid-heavy costs: Boggart Ram-Gang says "play RG" pretty clearly, because it's sooooooo much better if you can drop it on turn 3. But that only really works for aggro decks. You could do the same thing with a creature with a dramatic "level up" like monstrosity costing . But that's not really a mechanic.

So here's an attempt to fulfil the same design space in a scaleable way.

I think it sucks as a mechanic -- "bad hexproof" isn't fun, and "any sort of hexproof" is too non-interactive to be a major (even small) mechanic. But I wanted to start off with something simple and see if it nudged me to come back with any other mechanical ideas which were more fun.

It does seem appropriate. I agree that if this was the Simic ability in Ravnica, I'd be a little disappointing, but it isn't a bad take on an evergreen ability. More appropriate to a set that just happens to link blue and green, but not necessarily the other 9 colors.

Also, I think I personally would have made the blur more difficult to process in a three color deck, by doing something like "Blur 2 - " or so... but it's certainly fine as is.