They both take you to the next level. I kinda tried to clarify that in the text at the start of the level, but maybe I just made it more confusing, sorry.
Rather than be confusing, entering the black square on that level just plays the level clear sound without actually advancing to the next level.
Oh snap! I messed up the win condition at the last minute. Fixed that now (as of 7h3min post-deadline)
