See Phasing Specter and Goblin Pathfinder. Evade is an attempt at a mechanic that could appear on cards. An alternate possibility:
Evade N (~ can't be blocked except by N or more creatures.)
Evade ∞ (~ can't be blocked by creatures.)

Seems fine. I don't think it meets all the desired criteria for a new blue keyword mechanic, because blue already has an abundance of evasion abilities, but it's fine nonetheless.

It's hard to come up with an interactive blue combat mechanic because blue doesn't really want one.

Oh, indeed. We designers want it to have one, but its pie and flavour don't really want one. I had a discussion with lpaulsen on his blog post on blue creature keywords about what such a beast might look like... I ended up wondering about "mechanics that work better if you're smarter", but that's a hard flavour to turn into mechanics itself.

The idea behind Evade is that it would keyword "can't be blocked," so it actually wouldn't be an entirely new blue ability.

I'm thinking selectional evasion like deathtouch and a rider upon hitting opponent. so opponents get a bad deal whether they block or not. eg red gets Paintouch (whenever this blocks or is blocked, it deals X damage to that creature) and blue gets Weaktouch (whenever this blocks or is blocked, that creature gets -X/-0 until end of turn.) coincidentally both abilities also work in black, the color between red and blue.

riders are triggered when it damages an opponent, like draw a card, loot, exile-cast, steal an artifact, etc. so you get a benefit whether it's blocked or not. more interactive than pure evasion.