I've had varying success with Plunge into Darkness, especially with Death's shadow.  I usually run it as a 1 of replacing Darkblast depending on my meta at the time.   

I agree that Death's Shadow is a bit weak.  Here is an unpowered version of the deck I've been running recently with great success.  

Artifacts

1x Helm of Obedience
1x Lotus Petal
1x Null Rod
1x Pithing Needle
2x Hex Parasite

Creatures

4x Dark Confidant
4x Vampire Hexmage

Enchantments

4x Leyline of the Void
1x Necropotence

Land

1x Strip Mine
12x Swamp
3x Wasteland
2x Urborg, Tomb of Yawgmoth
2x Dark Depths

Spells

1x Darkblast
1x Demonic Consultation
3x Diabolic Edict
1x Vampiric Tutor
4x Dark Ritual
1x Demonic Tutor
4x Duress
1x Grim Discovery
1x Sadistic Sacrament
3x Thoughtseize
1x Yawgmoth's Will

Sideboard:

3x Null Rod
1x Pithing Needle
3x Yixlid Jailer
1x Chains of Mephistopheles
1x Engineered Plague
1x Darkblast
1x Echoing Decay
1x Ravenous Trap
3x Sadistic Sacrament