We did an Iceland visit on Summit. The curtain are black out. . . but just like all curtain leak light at the top, bottom and where the curtains join. A couple clothes pins will remedy where they join. Almost anything pretty in the bottom corners most of the light from the bottom and sides. Nothingvyou can donabout the top.
We were quite pleased with the limited light remaining. You might find it great, just ok or not acceptable. Many on board wore masks.