you do not maintain the anytime dining bookings you may have made pre cruise.
Blu is all open seating. No reservations are needed or taken unless you have an an usual situation like a group that all wants adjacent tables.
If you book Aqua, you may see on the app a place where you can make nightly dinner reservations. This is an artefact of a previous booking system and is not relevant to Blu. Even if you fill all of them out and you think you have made nightly reservation requests, you will find when you go to Blu that you have not.
Now on a night or two, that you might want to go eat in the MDR, simply tell the blue maître d’ and they will fix it for you. Remember that you don’t have a dining assignment in the MDR, Neither in the select dining nor in an assigned table in fixed seating. They will find you a table if you ask for one. It might be in select, or it might be in the part of the dining room given over to fix dining. It will depend on how the crowds are going where it might be easier to seat you.
They try to keep things balanced out so that those who have signed up for select dining don’t have too long a wait. Adding extra people in from Blu might not be feasible on a given night so they would have you sit in the fixed dining area that night.
it’s all a balancing act and keep in mind that they can’t move people from the MDR to Blu to take your spot there.