Void does not keep the passive benefits of Mom's Box if absorbed.
Synergies
Blind Rage: Grants much longer invincibility after taking damage than normal.
Cancer: Grants -4 Tear Delay instead of -2. When Isaac's tears stat is at its maximum, resulting in 5 tear delay, this synergy reduces tear delay all the way down to its minimum of 1.
Cracked Crown: Grants -2 Tear Delay and further increases damage, speed, range, and shot speed if they are above base value, by about 77%.
May be a reference to the film Seven where the climax of the film involves the protagonist asking the question over and over again "What's in the box?".