Two men were disputing. One said, "There are blue roses." The other declared, "There are not blue roses."

What did the first one assert about blue roses? What did the second one deny? You see the first one might have said, Blue roses exist.

Analyze the last sentence. You will see that the word exist is both copula and predicate attribute of existence.

The thought of the sentence may be expressed in this way also: Blue roses are. Here the word are is used like the word exist.