Blu is for Aqua class cabins only with the only exception being suites if space if available. Suites pay $5.00 PP and on some cruises it is available and others it is not. They also are very strict about only AQ cabins eating in Blu. If you are traveling with someone who does not have an AQ cabin they cannot eat in Blu with you unless they are in a suite. This is applicable at Breakfast also.
Any child who is staying in an AQ cabin can eat in Blu however this is unsual since AQ cabins are strictly limited to two people in a room. A child who is staying with their parents in a suite can also eat there.