When someone is said to be "having blue devils," it means they are feeling sad, downhearted, or melancholic. However, there are several antonyms that can be used to describe the opposite of this feeling. For instance, if someone is "on cloud nine," they are incredibly happy and elated. Similarly, if someone is "overjoyed," they are thrilled, enthusiastic, and cheerful. "Ecstatic," "blissful," and "radiant" are other antonyms that can be used to describe a person who is feeling the opposite of having blue devils. These words reflect different levels of happiness and joy, and can be used to express a range of emotions.