We were on the Solstice last week in an AQ cabin an ate in Blu all seven evenings. It IS all open seating now, but it was never crowded my guess never more than 50% of capacity. That said, we did get there every evening when they opened the doors at 6PM (we are early eaters). Being there at 6PM also meant that we got a table by the window every evening (there are about 6 table next to the windows, but many others very close to the windows).
As for your exact question: DO NOT SELECT "MyTime Dining" as that will place you in the MDR in their open seating section not in Blu. The last time I checked our booking before departure the Dining Selection options where still traditional or MyTime. Select traditional (early or late). The only reference to early vs. late was on the first night and there was a note on our bed that requested that we dine the first night in our selected time slot (early in our case). After Sunday evening, all is wide open to AQ passengers. (Actually, I am not even sure that they enforced the early/late request the first night. I think they just wanted to make sure that there was no rush to the door at 6PM, which never happened any evening.)
05/89 NCL, Seaward, 1990 Carnival, Topicale, 1991 Carnival, Festivale, 11/93 Wind Star, Wind Song, 01/95 Celebrity, Horizon, 01/96 Celebrity, Century, 01/00 Celebrity, Zenith, 01/01 Princess, Ocean Princess, 01/02 Princess, Ocean Princess, 01/03 Celebrity, Summit, 05/04 Princess, Diamond, 01/05 Celebrity, Galaxy, 01/07 Oceania, Regatta, 01/08 RCCL, Voyager of the Seas, 02/09 RCCL, Voyager of the Seas, 01/10 Celebrity, Solstice