Ristin's Campaign Construction Center
The idea that immediately springs to mind for a Black Crusade game is for a first-founding squad of space marines who would then live-out a series of past and present encounters. As effectively ageless chaos space marines they could be part of a truly epic story arc spanning from before the Horus Heresy up until the current time. From their origin as loyal space marines fighting alongside the Emperor, through the fall of their chapter under Warmaster Horus, then snippets of the thousands of years hence (major events only, naturally) and periodic squad reunions.
That’s a concept, anyway. It would take a lot of GM work and then character participation (and backstory) to even attempt such a project, fleshing out the character’s human-life origins, their induction as space marines, their rough career and choices through the fall and then their reactions to life as chaos space marines.
Would they be regretful, eager for redemption? Would they be loyal to the ’liberator’s’ cause against the Emperor and pursuing plans against the Imperium? Or some personal project like seeking immortality through demonhood, or perfecting the arcane arts, or something else?
Possibly a safer approach would be to begin with a Deathwatch game set during the Horus Heresy with the players knowing they’re in a chapter that goes heretic and play that out, then run the Black Crusade game as a sequel? This would have the advantage of the “un-corrupted” character sheets to work from later.