The Negative

The Negative is an unlockable passive item.

Effects

If the character has half a red heart or no red hearts after taking damage, it triggers The Necronomicon effect, damaging all enemies in the room.
Allows entrance into the Dark Room after beating Satan in Sheol by touching the chest.
+1 damage.
Opens A Strange Door in Depths II and its variants that leads to Mausoleum/Gehenna II.

This item is a guaranteed drop from Mom alongside The Polaroid, provided they are unlocked. However, they are mutually exclusive: Once one of them is picked up, the other one will disappear.
- With the use of a teleportation item and either a 5 pip Dice Room or a Forget Me Now, both The Polaroid and The Negative or multiple copies of The Polaroid or The Negative can be obtained.
The Negative always triggers when taking damage as characters who lack heart containers, such as Azazel and Tainted Azazel (by default), ??? and Tainted ??? (permanently), Dark Judas and Tainted Judas (permanently) The Forgotten (only as the soul), Tainted Forgotten (permanently), Dead Tainted Lazarus(by default), Tainted Bethany (permanently) or any other character who has chosen to trade away their heart containers in deals with the Devil, Black Markets, or with Guppy's Paw or Potato Peeler.
The Negative will be the rightmost item after defeating Mom, in case Curse of the Blind is active.
It is possible to obtain two Negatives by obtaining one as an Eden starting item.
Obtaining multiple copies of The Negative does not cause its effect to stack.
As with all items crucial to story progress (Key Piece 1 and Key Piece 2, Knife Piece 1 and Knife Piece 2, Dad's Note, and Dogma), this item has protection from full-run rerolls. It does not, however, share the immunity Dad's Note has to D6-type rerolls and Void.

The Negative allows access to the Dark Room, which is also the name of a room used to develop photos (Darkroom).
A negative is a piece of film that's been used to take a photograph but hasn't been developed into a finished photograph. A negative has inverted hues as well as inverted lightness.
- Because of the instant nature of Polaroid film versus regular film, however, one would usually never see the Negative of a Polaroid and a dark room would not be required to develop one, which calls into question its relation to The Polaroid. However, this could probably explain the fact that both items can't be picked up together in a run (unless Forget Me Now or a 5-pip Dice Room is used).