The /dev/null is a tool from the OpenBlocks mod. This tool is used to discard all of a single item type when they are picked up. It has a single inventory slot, which can be accessed through using it with it pointed at air; it cannot be in reach of another block. Sneak+using it will also work to open the GUI. The item that is put in there is the "filter" item. Once the stack in that slot is full, all exact items will be immediately discarded on pick-up. You can use the tool with a block in it to place the block.
With a block in its slot, the tool can be used to place the block.