UA919 (LHR->IAD) yesterday in BF. I placed my blanket (still wrapped) on the ottoman prior to takeoff. I briefly dozed off during taxi/takeoff (yes, I can sleep during takeoff), and when I woke up, the blanket was "gone". I was in one of the four middle seats, and I have no idea if someone lifted it. After meal service, I got up and asked one of the FA's if they had any extra blankets, explaining the "disappearance" of mine. She said they didn't have extras because BF was full, but would try to find one that the passenger wasn't going to use. I politely told her it wasn't necessary and returned to my seat, but a few minutes later, she came by with one from GF. When deplaning later, she jokingly commented about the disappearing blanket.
Still no idea what happened to it. I looked all around the seat and never found it.