Learn German: Das Auto ist blau.

Because blau comes after the verb ist. In German, adjectives only get matching endings when they are placed directly in front of a noun (as in das blaue Auto). When they describe a noun but come after the verb, they always stay in their basic, unchanged form.

Is there a trick to remember why it is das Auto instead of der or die Auto?

Auto is a neuter noun (das). A handy rule is that many foreign words or technical terms ending in the letter -o are neuter in German. Other common examples include das Kino (cinema), das Foto (photo), and das Radio.

How does this sentence change if I want to say "The cars are blue"?

It becomes Die Autos sind blau. To make Auto plural, you just add an -s, which is the standard rule for German nouns ending in a vowel like -o. Notice that the adjective blau still doesn't get any ending!