So I've seen a few people point out the room, but I've yet to see anyone explain how to open it. After a bit of testing I THINK this video is correct, though at this point who really knows.
What I can say is having Blood Moon Essence is not a requirement. I specifically joined another world on this stage and did not get the door to open, which is where the middle video came from. I can also confirm killing the boss is not required. I did a run with Rubhen and it did not open after the boss died. On the run it did open the boss was not even engaged. I can also confirm it isn't some kind of trigger as we both messed with it a bit and got nothing, and you can even see in the video it just kind of happened.
Unfortunately, I don't have the time to test getting another Blood Moon and closed room, but that seemed to be what worked. I basically went to this location, found it, left, finished two more, saw a Blood Moon, tried it, and it randomly opened like shown.
Hopefully this helps or at least makes it easier to understand/obtain.