This is meant to represent the most simple primary secondary common, using existing Magic mechanics.

Black and Red get firebreathing/shadebreathing. This isn't found on Blue.

Vigilance is a keyword that only Blue gets out of the three.

There are Black and Red Goblins, and Warrior is primarily a creature type in those colors. I'm not thrilled about this exact set, though, because it feels like it isn't representing Blue enough. Probably want a distinctly Blue creature type on that line. Blue also gets some Warriors, so it's not entirely using the supplemental colors.

Unsure what rate to put the shadebreathing. Is it really "red" if it also increases toughness? Can we sneak haste in instead of the toughness increase?

I want this to remain "common" in complexity... but how can we make this feel more Blue? Is there a way to tie in the -breathing to some kind of output that is rather blue?

We could have it Scry X, where X is the damage done?