Not sure. I never do TF because all i can ever seem to find are those stupid little houses. The blocks are called Reappearing block and Vanishing block. There's an invert of the Vanishing block. You right click them and the make any other vanishing block disappear for a short amount of time
The reapearing and vanishing blocks are all found around the Dark Towers in the Twilight Forest. They make some pretty awsome doors.