Blackshield
A Blackshield is a Space Marine who has forsaken their former chapter. The chapter, assuming they still exist (and are aware of the Blackshield in question), 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. Most Space Marines (barring certain examples) know and acknowledge their own worth in the battle against the Imperium'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 a case of a Blackshield Watch Commander are not unknown.
It has recently been confirmed (in one of the Horus Heresy novels) that the predecessors to the tradition of the Deathwatch Blackshields stems from the Horus Heresy. During the Horus Heresy, loyalist marines from traitor legions who shunned their traitorous brethren and remained loyal (or in rarer cases were from loyalist legions but turned traitor- or rarer still from the ones who went "fuck it" and decided to strike out on their own without taking either side) would remove all livery that connected them to their legion, and were referred to as Blackshields.