Blue doesn't get damage prevention. It can grant -X/-0 though.

Formerly: Vanishing

Why doesn't blue get damage prevention? White gets it because they're the healing and damage prevention color, so that makes sense. And blue? The color of illusions, manipulation, and evasion. Seems perfectly flavorful to me.

Flavor doesn't matter. Blue isn't allowed to do that mechanically.

But what determines mechanics? I thought flavor. Alpha was designed top down, not with flavor as some actual design policy but because in most cases it made sense. Birds of Paradise, Black Knight, and Giant Spider are good examples.

Also the entire flavor of the colors of Magic is grounded in (primarily I assume) Western notions and associations. Why does red get the mechanics like direct damage and haste creatures and chance effects? I wonder how people who've never played Magic would describe a particular color...

"But what determines mechanics? I thought flavor." - Many things. The flavour of the way the colour plays overall should match the flavour of that colour's philosophy. Each colour needs to have certain mechanical weaknesses. You're right that Alpha was designed top down, and that led to a number of cards that are way outside the modern colour pie.

Blue's weaknesses are (and always have been I believe) its creatures and lack of speed (outside artifacts). It doesn't get the splashiest, bombastic creatures. They can't (shouldn't be able to) go toe-to-toe with the other colors, requiring blue to have evasion, control spells, and combat tricks to back it up.

Don't think of this as prevent damage but avoid damage. Part of blue's toying with reality. It's like the planeswalker (player) blinked out of existence for a moment, and then comes back with http://mrwgifs.com/wp-content/uploads/2013/11/Cant-Touch-This-MC-Hammer-Dance-Gif.gif

Also, doesn't it make sense from a thematic, flavor, and mechanical standpoint? It's outside the modern colour pie, sure, but who says the modern color pie is right? Or even in its final form?

If you want to make up your own mechanical identity, fine. That's not really Magic, so I won't be able to help. If you actually want to make something that resembles a potential future set, you need to match the actual color pie, which has years of settling into its current form.

If you want this card to exist within the color pie, make it do something blue is allowed to do. For example, : "Creatures target player controls get -X/-0 UEOT", or : "End the phase. Cast only during combat."