Step by step guide. This assumes you are playing offline, kill yourself in the nexus each time you revive and all worlds are neutral upon game start.
World 1 (boletaria)
Pure White: Kill 3 bosses (dragons counts as bosses). -- The Blue Dragon leaves the roost, and Miralda appears in the execution grounds.
Pure Black: Kill Miralda (at pwwt), Old King Doran, and kill yourself two times. -- BP Miralda shows up, the Primeval Demon appears in 1-3 (so make sure you can access 1-3 before killing BP Miralda), and Biorr will attack you at the end of 1-4 (if you freed him).
World 2 (stonefang)
Pure White: Kill 2 bosses. -- Scirvir shows up in 2-2, and the Dragon Bone Smasher can be picked up in 2-3. Show the DBS to Scirvir to get a Pure Greystone.
Pure Black: Kill Scirvir (at pwwt - kick him off the ledge to avoid ct change) and kill yourself 4 times. -- BP Scirvir shows up in 2-2, the Primeval Demon appears in 2-1. Kill BP Scirvir first (or make sure he has spawned before you kill the PD) since the Primeval Demon shows up before Scirvir.
World 3 (latria)
Pure White: Kill 2 bosses. -- The rubble blocking the path next to Rydell disappears in 3-1, and the key to free Rydell appears in 3-2. Free Rydell (let him live) and he will unlock the last wing of the prison (close to the first crystal lizard).
Pure Black: Kill yourself 7 times. -- BP Rydell shows up in 3-1 (kill him first since you can't go back from 3-2 to 3-1!), and the Primeval Demon shows up in 3-2.
World 4 (shrine of storms)
Pure White: Kill 2 bosses (vanguard doesn't count) -- Satsuki appears in 4-1 and the Makoto can be picked up in 4-2.
Pure Black: Kill Satsuki and kill yourself 4 times. -- BP Satsuki shows up in 4-1 and the Primeval Demon shows up in 4-2.
World 5 (valley of defilement)
Pure White: Kill 2 bosses. -- The ladder to reach the Estarelle appears in 5-1, and Selen appears in the swamp (5-2). After you kill the last boss BP Garl will appear in the boss area, drops the Crest of Vinland. Give the crest to selen (during pwwt) to get a Ring of Devout Prayer.
Pure Black: Kill Selen (right after completing her quest) and kill yourself 4 times. Alternatively kill yourself 7 times to avoid ct change. -- BP selen appears in the swamp and the Primeval Demon appears in the swamp (close to the shanty town).
-You can always achieve pure black tendency by killing yourself in body form 7 times from pure white.
-Kill the BP npc before you kill the primeval demon, since the primeval demon will appear one shade lighter than black, to prevent any screw-ups.
-Killing the white world versions of npcs (apart from miralda and satsuki) will also drop your character tendency. You can kick them off a ledge, use soulsucker, poison cloud or death cloud to kill them without changing character tendency. Beware that killing the BP versions the same way will also prevent a character tendency change towards white.
-Do not go back to the Nexus before you have killed both the BP npc and the Primeval Demon in a black world, since this will shift the tendency back towards white.
I usually complete the white world tendency events in a world or two, then suicide there after I kill a boss in another world to help shift it towards black without using too many ephemeral eyestones.