Is "I could not believe my eyes" an interjection

No, that statement is an example of an idiom.

Explanation: an interjection is a word or a phrase (or could be a sentence) that expresses emotion or feeling. Example: Congrats!

explanation:

if you’re a fan of chocolate, every occasion is right for this treat. feeling down? eat some chocolate. celebrating? eat some more. want to show your valentine how special you think your sweetheart is? offer up chocolate!   people have been enjoying chocolate as a food, drink and medicine for thousands of years. the ancient maya and aztecs called chocolate kakaw and used it as medicine. they also made chocolate offerings to their gods. chocolate comes from the cacao (kuh-kow) tree, and its scientific name reflects its history. theobroma cacao translates to “cacao, food of the gods.” these days, chocolate is not just for deities. the treat has become popular around the world.