"Er ist völlig blau."
Wenn du lernst eine neue Sprache, musst du ein anderen weg denken zu lernen.
Do not try to make English and German match together, each one is what it is, and works how it works. And that's the thing with EVERY language, they work much more or a little differently from each other. For example, the expression "black eye" (when someone gets hit right to the eye with a heavy fist)
English: A black eye ("BLACK EYE") German: Ein blaues Auge ("BLUE EYE") Spanish: Un ojo morado ("PURPLE EYE")
You see? And is sounds perfectly right and normal the way it's said in each language, because they work differently, BECAUSE they're different languages, and it's perfectly right. So, when you learn a new language, have in mind you're also learning to think in an entirely different and new way.
I hope I was helpfull. Grüßen aus Mexiko!
From my understanding duolingo is about language as it is useful in everyday life. Only the fewest people are actually blue colorwise. Therefore the translation given here (völlig blau / totally blue) is academically correct but not useful in everyday life. If you want to teach the different meanings of blue/blau then you should give the everyday translation. If you do not want to focus on these special meanings, then use a different color instead (e.g. red).