the key was the notion to have a block "focused" even if it moves, so we can average multiple instances of it (if we're luck enough that block didn't changed much over time). 
that brings us to downside; if content inside the (moved) block is changed in present block, then we're averaging two different things(different contents of block), and we're introducing blur(ghosting) to that particular block, and if you have one block that's blurred amongs other blocks that look normal(these got exactly the same content even when their blocks were moved, or simply they didn't need to be moved at all), you have visible blocking.