You'd think that this word would use on'yomi readings to read it, but it doesn't (shake that angry fist at the kanji gods!). Instead, it's double kun'yomi, probably because this word is really like two separate words put together, "blue" and "sky." In the previous level you unlocked these two separate words, learning their readings. That means you should be able to read this as well.
One quick note is the rendaku-fication of 空 as well. It becomes ぞら.
Is the ocean blue because of the blue sky?
Let our future generations have the same beautiful sky as we do.
Today, it was sunny all day and the blue sky was above us.