In UK English, normally you go red, or in this case, they went red - but it is marked wrong. To turn red happens to tomatoes or cherries or whatever else changes colour as it ripens (because they don't go back to being green, but people normally change back after they flush... turning a colour normally implies permanence in UK English).
Duolingo usually accepts UK spelling, with some exceptions (i.e. Brasil) but often disregards phrases that are different from American English. It feels as though those writing the courses have a strong American inclination. I would report it, but I doubt they would bother to add additional answers.
It's an irregular verb in the past tense
to become = at blive
becomes = bliver
became = blev
have become = er blevet
Thanks. It seems confusing that "at blive" can both signify a change and no change at all. As far as I can tell "I turned red and I stayed red" would be "Jeg blev rød og jeg blev rød". Or are other verbs used in that context?
I am not an English native but I think that blush is mainly used for people, whereas turn red also applies to objects of all kind: The tulips turned red - they turned red, but they did not blush.