The following cards in black and green will allow you to search your library for a sorcery, and possibly other cards:

 - [mtg:Bringer of the Black Dawn]
 - **[mtg:Cruel Tutor]**
 - [mtg:Demonic Collusion]
 - **[mtg:Demonic Tutor]**
 - **[mtg:Diabolic Intent]**
 - [mtg:Diabolic Revelations]
 - [mtg:Diabolic Tutor]
 - **[mtg:Entomb]** (tutors to the graveyard)
 - **[mtg:Grim Tutor]**
 - **[mtg:Imperial Seal]** (tutors to the top of the library)
 - [mtg:Increasing Ambition]
 - **[mtg:Infernal Tutor]**
 - [mtg:Insidious Dreams]
 - [mtg:Liliana Vess] (tutors to the top of the library)
 - **[mtg:Rhystic Tutor]**
 - [mtg:Rune-Scarred Demon]
 - [mtg:Sidisi, Undead Vizier]
 - **[mtg:Vampiric Tutor]** (tutors to the top of the library)

I've highlighted the cards with converted mana cost 3 or less.

There are also black cards with Transmute, which allows you to pay {1}{B}{B} and discard that card to search for a card with the same CMC:

 - [mtg:Brainspoil]
 - [mtg:Dimir House Guard]
 - [mtg:Dimir Machinations]
 - [mtg:Netherborn Phalanx]
 - [mtg:Shred Memory]