ebolazaire said:
p.319 of the corebook details the Blackshields.
They are essentially Marines that for some reason or another no longer has a chapter, and opt to join the Deathwatch for life.
The book gives a few examples like being the last marine alive of the chapter, turning from your chapter [chaos cult] or whatever, if I remember correctly they give an example that hints very heavily that the marine is one of the Fallen that repented.
Hello. Source of blackmail right there.