when you click on the block, copy it's UID to a variable (SelectedBlock). When you click on an empty block copy it's UID to a variable (EmptyBlock). After your click move SelectedBLock to EmptyBlock. Or you could just capture their X,Y coordinates. So On click empty block store the X,Y in the emptyBlock variable, and on click the SelectedBlock copy it's X,Y. to the SelectedBlock variable. Then just set selecteblocks X,Y to the coordinates stored in emptyBlock.
You would also need to check if you can move to that square though.