What is the significance of having 0 health and the last health slot shaking, exuding black spots?

I think I can only get this when I have Joni's Blessing charm equipped, and when I lose my last blue health mask. It seems like there is this one final chance given to you when you lose all of your health points, you can still take one more hit, but the health slot is empty, shaking and emitting black spots. Those look like the same black spots that your death shade emits.

I just won a fight against a dream boss (Lost Kin) and reappeared with full health, but my body was fuming with black spots and the left-most health slot was shaking and emitting black spots too. Although I'm assuming this specific scenario was not accounted for in the game's code and wasn't intentional.

Is there any significance to this lore-wise? Or any explicit explanation about why you can have this one extra hit point? And why it causes you to emit black spots?