Learn Italian: Il mare è blu. - The sea is dark blue.

In Italian, the word blu specifically refers to a darker, deep blue. For a lighter, sky blue, Italians use a completely different word: azzurro.

Does the adjective blu change its ending if the noun is plural?

No, blu is an invariable adjective, meaning it never changes form. Whether you are describing one sea (il mare è blu) or multiple seas (i mari sono blu), the word stays exactly the same. This is also true for a few other colors like rosa and viola.

The word mare ends in -e. How can I tell if it is masculine or feminine?

Nouns ending in -e can be either masculine or feminine, so you usually have to learn the gender along with the word itself. In this sentence, the definite article il gives it away and tells us that mare is masculine.