Blackshield
A Blackshield is a Space Marine who has forsaken their former chapter -- the chapter, assuming they still exist, has completely disavowed the marine in return. These lost souls remove any heraldry or relics from their person which have any bearing on their history, blanking it completely and leaving it a plain black. They then petition to join the ranks of the Deathwatch on a permanent basis.
In a bit of a "don't ask, don't tell" parallel, the Watch Commander doesn't ask the applicant's history, knowing only their name and skills. Whatever does end up passing between them is forever a secret.
These guys are rare - the special snowflake of the Space Marine special snowflake Alaska. I can already hear you Deathwatch players rolling up your next character as a mysterious Blackshield with a mysterious past. Pfft.
No two Blackshields have the same motivations for being one, but what is common is that they are all zealous individuals, wishing only to earn their redemption via the destruction of the foes of the Emperor. There are an abundance of reasons why they join - maybe they were part of a chapter that went full blown heretic and went all survivor's guilt; maybe they lost some incredibly important relic or got a good portion of those under their command killed... or maybe they were just refusing an order out of basic fucking common sense purely because it was a stupid thing to do in the situation.
These guys are suicidal fucks. Every Space Marine knows and acknowledges their own worth in the battle against the Emperium's enemies - not the Blackshields. They'll think nothing of jumping headfirst onto a Melta Bomb or drawing enemy fire. In a Blackshield's mind he is as good as dead already, and he will always go forth for the most dangerous missions without a second thought. On the flipside, he will perform duties deemed too unimportant or lowly for a Space Marine to perform without question, such as guarding secure outposts alone.
Whether this redemption comes in death or in chalking up enough Ork heads or Eldar scrotums is anyone's guess, but a very rare few do this with the intentions of being accepted back into their former chapter. Most are just happy to get a proper burial with full honours at one of the huge fortresses of the Deathwatch. Still, some of them end up rising throughout the ranks of the Deathwatch itself - Blackshield Deathwatch Chaplains, Watch Captains and even one Watch Commander are not unknown.