"Et blåt dyr."
Translation:A blue animal.
From the intro to the lesson (and modified to suit this color):
When used to describe a neuter gender noun such as "blod" the adjective is suffixed with -t and becomes "blåt".
Take a look at this discussion thread to see some examples of Danish common gender and neuter gender words.
You can only see the intro on the website, not in the app. I love duolingo but this fact is easily the worst thing about it.