On Oasis, there was staff in Boleros taking reservations. My understanding is that Royal maintains 25% for on bord bookings. Recommend that you do it as soon as you board. Also, you can go on a standby basis and 10-15 before performance they will allow you in assuming that there are seats available.
For example, on my October 8 Oasis sail I couldn't get any reservations for the Adult Show.
I went to the standby line and the line in essence was larger than the actual reservations line and subsequent late arrivals. At about 10 minutes prior, the entire standby was allowed in. The Blaze room is on an odd shape and includes a bar that one can sit and enjoy the performance.
Ps. You may also be able to do it on the APP, upon boarding.