At the end of the game, after you have finally caught up to that elusive black cube of death, you begin to suck it up into your gun.

Begin sucking up the black cube then before you fully capture his essence hit Esc.

You will now be back in the main menu and your gun will be black.

It shoots black cubes and acts exactly as the red gun.

This feature was entirely unintentional by the dev and is just a awesome bug/easter egg that I discovered. The only drawback is that it is hard to see where you place your cubes on top of other black cubes since you can no longer see the outline of your box placer and the black all blends in together.

Upon exiting the game, your gun reverts back to normal