What are the Blackshields? Ive honestly never heard of them.
This exactly what I was alluding to in an earlier post. Blackshields are Deathwatch Marines that have chosen to wipe all external signs of their previous chapter affiliation, and they mysteriously appear in front of the Watch Fortress. They will become Deathwatch Marines for life, in exchange for which they are not asked why and how. I believe it to be a FFG addition to the setting.
In that very sense, joining the Deathwatch is like joining the French Foreign Legion - or the Templars. All is forgiven, if only you fight for us.
As an aside. Of course the Space Marines are all we wish to be. Righteous, clear goals, nothing complicated but just fight for the emperor. Not bribed, bought, scared of or turned. Unsullied.
There is this tiny tiny black spot in their history. The heresy. Whole chapters turned to Chaos - In the Chapters, even some infighting occurred. The Dark Angels still, thousands of years after this, are on a fools errand to find all the heretics. Some Space Marines even run away from their perfect lot to become corsairs.
Unfortunately, even the Space Marines can not entirely be relied on - in the space of thousands of years, some of them listen to the ruinous powers, others rebel against the empire, whole chapters becoming renegade.
Is it really that difficult to imagine that, given the right circumstance, something like an illicit child mike ensue and shame the Marine into becoming a Black Shield to atone, and then be blackmailed with that child ?
Some have suggested this train of thought as wrong, maybe even offensive. I realize the value of concepts brought forward in the long tradition of Games Workshop products, and I am certainly aware of how attached some of us become to certain notions of heroism.
Still, for the purpose of this roleplaying game, and the question of the original poster, I don't think the idea of the blackmailed marine borders on content that require it to be renounced on grounds that it doesn't fit the IP - That IP has been bent and molded to suit any current market trend for the past thirty years, and I think can stand bending a little more by me without breaking.
See Black Shields sit abit less than well with me but are understandable. There are examples throughout the fluff of space marines losing sacred relics (Bloodquest), causing the death of superior officers (Rynn's World) and turning traitor then redeeming themselves (Babab Crusade). In all of these a punishment is handed out be it a crusade of penance or being mind wiped and turned into a servitor. Thusly it is reasonable for a Space Marine who believes he has gravely damaged his chapters honor and feels that his punishment is not enough and so removes himself from his chapter to devote himself to the Deathwatch as a blackshield. They may be the last of an ill fated chapter, a marine who survived a battle none of his brothers did or a member of a group which turned to chaos where he stepped back before it was too late. It is even conciveable for there to be Chaos Marines who haven't gone too far and wish to repent (Alpha Legion would likely be a possible source of these). But the point is that if there is something worth leaving all their past behind the Blackshield would quite likely go to great lengths to keep it covered up, playing along until he can break the blackmailers neck. Also its worth bearing in mind that most Space Marines would more than likely admit, if not what they did, that someone is trying to blackmail them.
Also the Heresy is a completely different issue. Its less a bunch of Space Marines leading a coup and more to do with the Primarchs. As far as has been revealed in the Horus Heresy books it was far from black and white. Spoilers ahead.
Lion El'Jonson is still a grey area as to what happens with his chapter (its looking likely Luthor is the one to turn to Chaos but the Lion is still being a bit of a jerk), Fulgrim was tricked and possessed by a demon through his vanity, Night Haunter was feeling alienated from the Emperor, Angron never truely forgave the Emperor and was already turning his sons into berzerkers before the heresy started, Mortarion didn't know about the Chaos side of things and believed Horus would lead to a greater destiny for humanity, Magnus was forced to switch sides by the Emperor casting him out despite trying to prevent the whole thing, Horus himself was manipulated towards this course of action for man years, Lorgar was feeling betrayed although its looking like there's more to his story than we yet know (First Heretic isn't out yet) and Alpharius is working a big picture thing for the betterment of the Galaxy. For the Space Marines themselves it was more a case of choosing between their distant grandfather and their father who was their with them. Even then there were those who remained loyal (Galaxy in Flames, Flight of the Eisenstein, Fulgrim and Battle for the Abyss feature loyalist remnants of the traitor legions).
So yes whilst it is possible for a Space Marine to go traitor it doesn't mean that other things are likely to happen. In particular an illicit child is just inconciveable. For one thing the logic of Man of Steel, Woman of Kleenex comes into play and for another these are beings who are so modified on a genetic level that it is near impossible to imagine. The main issue with the whole illicit child deal is it just can't physically occur. Space Marines are recruited to young to have concieved (if I remember my fluff correctly its something to do with puberty greatly aiding the addition of the geneseed and they're recruited long before thats implanted) and Space Marines are just that incompatable with ordinary humans. And this is ignoring the fact that for most ordinary humans to even see a Space Marine is unlikely and even generally when they do it is either because they are going to become a Space Marine or because things are about to hit the fan. When a Space Marine unit arrives in a civillian area it is usually more likely to be considered a religious event than anything else. In Rynn's World the Governess of the planet is attracted to the Crimson Fist chapter master but understands that there can never be anything there. They are that removed from humanity that the only time they see them is when they are engaged in deadly combat.