Yes, this is an error of Duolingo.
Łizhin and other color words are verbs, meaning "to be + color". So Shash łizhin means "bear is black".
To say "the black bear" in Navajo, one needs to turn the verb to a relative: "the bear that is black". To do so, one can add -í, -ii, or -ígíí to the end of the verb: Shash łizhinígíí.
That is also how "orange" the fruit is constructed: ch'il łitsxooí, is actually "the plant that is orange", with łitsxo meaning "to be orange" and -í the relativizer.