Black creatures that cause loss of life when they enter the battlefield?
Technically, there appear to be only three black creatures that deal damage when they enter the battlefield, and for color pie reasons they're all black/red: Anathemancer, Hellhole Rats, and Murderous Redcap.  However, if, as your example indicates, you're also after creatures that cause loss of life (which doesn't count as damage; this matters for things like Curiosity), you also have available:


"When ~ enters the battlefield, target player/opponent loses X life."


Geralf&#39;s Messenger
Inquisitor Exarch
Laquatus&#39;s Champion
Maga, Traitor to Mortals
Singe-Mind Ogre
Soul Scourge

"When ~ enters the battlefield, target opponent loses X life and you gain X life."


Bleak Coven Vampires
Bloodhunter Bat
Dakmor Ghoul
Ghost Council of Orzhova
Highway Robber
Obzedat, Ghost Council

"When ~ enters the battlefield, each player/opponent loses X life."


Blood-Toll Harpy
Howling Banshee
Maggot Carrier
Rathi Fiend
Netherborn Phalanx

"When ~ enters the battlefield, each opponent loses X life.  You gain life equal to the life lost this way."


Gray Merchant of Asphodel
Malakir Bloodwitch