When aiming your cursor on a block, the gray lines outline the block to indicate you are selecting that block. Well when you break the particles, the gray outline box actually renders over top of the particles. I will try my best to capture it using light colored blocks to create contrast with the gray selector.

The easiest way to see it in action is to go in creative and use white wool or quartz (for contrast reasons so you can see it easier) place about 5 stacked up and dig all of them, (below your feet) so you can see the next block selection overlay the particles