Reappearing Block is a new block from the Twilight Forest mod. It can be found in the Dark Towers in the Twilight Forest. These blocks, when clicked on, will disappear briefly allowing you to walk through where they were before reappearing again.
There is a technical block of the same name that only has the purpose of serving as the intermediate states of the Reappearing Block. This block cannot be obtained legitimately nor is it craftable, unlike the above block, and it is not used in any crafting recipes. It is transparent in nature can be walked through without hindrance.