Done.It's wrong then. I had already saved the Centaur and had the trophies for finishing Avalon I through 3 and the Centaur in Avalon IV was still blacked out. I was stuck on this for days so I clearly remember how it went. Only when I played the Centaur on Avalon I a second time and sacrificed did it unlock for me. I'm sure that's how it went for me.
You can just add it as a possible additional next step in case people find it necessary. Sifting through 40 pages just to find that bit of info you might very well need could prove a hassle (I know I searched everywhere with no luck).
How do I unlock the blacked-out missions in Avalon Pact IV (Atonement)?
Complete all of the missions in the Destinies Past story, sacrifice the Harpy in Avalon Pact I (Innocence) - Snowfield Glutton, and complete the Desert Drag mission
(Avalon Pact I). You may need to sacrifice the Centaur.