ONTARIO – From the shadows, the voice is easy but insistent. “What happened to the sleigh?” Sterling Blackwell said. Blackwell, hidden inside the darkness of the Meyer-McClean Theater at Four Rivers Cultural Center, watched Saturday and guided his young cast of the play “Elf” through a rehearsal. The musical, based on the 2003 movie of the same name, will open Thursday at 7 p.m., and shows again at 7 p.m. Friday and 1 p.m. and 7 p.m. Saturday. Tickets are $5.