Welcome to Cruise Reviews.
I can never keep this straight but Blu *is* your assigned dining room if you are in AquaClass. It is open seating for dinner although they ask that you arrive at a set time on the first night only. Blu is highly regarded and most people rave about it over the main dining room. They do have 'every day' items if you don't like the featured menu, just as they do in the main dining room. Plus, Blu is more intimate and has a true restaurant feel to it as opposed to the large, open, dining hall feel you get in the MDR.