Spawns a familiar that follows Isaac around and collects Red Hearts. For every one-and-a-half Red Hearts collected, Dark Bum will generate one of the following:
This item belongs to the Super Bum set. Collecting three items from this set will transform Isaac's bums into Super Bum.
Notes
If Dark Bum enters one of a set of Super Secret Rooms which allow only certain types of Hearts to generate in them, he will drop that type of heart instead of what he normally would.
The one which only allows Red Hearts will cause Dark Bum to consume the Red Hearts on the floor only to generate yet more Red Hearts. Leading Dark Bum around the room to consume the Red Hearts and then leaving the room before he's able to generate anything will avoid this.
Temporary hearts (such as from Tainted Magdalene, Tainted Eve with Birthright, etc.) can be collected by Dark Bum, which situationally may prove useful or dangerous.
Alternatively, The Forgotten or characters with low health may suffer from Dark Bum, as it will make it harder for The Forgotten to maintain Bone Hearts.
This is averted if Keeper is in a co-op party with other characters that can utilize Hearts.
Dark Bum may also be picked up by Keeper for the potential transformation.
Scared Hearts will run away from Dark Bum. Pinching them between Dark Bum and Isaac or walls/corners will make them easier for the Bum to collect.
From a scoring perspective, Hearts that Dark Bum picks up count towards your Schwag bonus on daily runs.
Dark Bum does not join the line of familiars that follow behind Isaac.
Synergies
Humbleing Bundle / Equality!: Most Red Heart drops will become Double Red Heart drops. This counts as two full Red Hearts for Dark Bum, essentially doubling the value of it.
This item is the Devil Beggar counterpart of Bum Friend and Key Bum.
Bugs
Bug!
Activating Glowing Hourglass twice after Dark Bum picked up Red Hearts and before he finishes his animation will save his Red Heart pick up progress while the Red Hearts will stay untouched in the room.
Red Hearts. For every one-and-a-half Red Hearts collected, Dark Bum will generate one of the following:
A single 
Soul Heart.
A single 
Black Heart.
Spider.
This item belongs to the 
Super Bum set. Collecting three items from this set will transform Isaac's bums into Super Bum.
Super Secret Rooms which allow only certain types of Hearts to generate in them, he will drop that type of heart instead of what he normally would.
Gold Hearts, 
Blended Hearts, 
Bone Hearts, or 
Rotten Hearts.
Tainted Magdalene, 
Tainted Eve with 
Birthright, etc.) can be collected by Dark Bum, which situationally may prove useful or dangerous. 
???, 
Tainted ???, 
Dark Judas, 
Tainted Judas, 
Tainted Forgotten), since they have no use for Red Hearts.
The Forgotten or characters with low health may suffer from Dark Bum, as it will make it harder for The Forgotten to maintain Bone Hearts.
Keeper / 
Tainted Keeper generally has no use for the Dark Bum since all Hearts are converted to Blue Flies.
From a scoring perspective, Hearts that Dark Bum picks up count towards your Schwag bonus on daily runs.
Humbleing Bundle / 
Equality!: Most Red Heart drops will become Double Red Heart drops. This counts as two full Red Hearts for Dark Bum, essentially doubling the value of it.
BFFS!: No effect, but familiar is bigger.
Dark Bum / 
Cambion Conception: Each Dark Bum tracks the collected hearts separately.
Options?: If one of the options is a Red Heart variant, Dark Bum may collect it, making the other option disappear.
Bum Friend and 
Key Bum.
Bug!
The Binding of Isaac: Rebirth
