"Night gathers, and now my watch begins. It shall not end until my death."
- – A Song of Ice and Fire
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 French Foreign Legion 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.
tl;dr They're Space Marine Slayers.
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, but got kicked out anyway for breaking the rules.
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 content to kill for the Emperor until they meet a glorious end, buried with honour 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 cases of Blackshield Watch Commanders are not unknown.
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, legionaries 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, referring to themselves to as Blackshields.
Despite breaking off from their parent Legion, many of these Blackshields were able to make more Space Marines as they have removed all restrictions on gene-seed production and freely experimented with it (resulting in many Blackshields having gene-seed from multiple Legions) to expectedly unpredictable results. However, these Blackshields were forced to scavenge for their own gear and potentially make their own slapdash armaments, resulting in Marines wielding lasguns (or potentially alien weapons) because their bolters were too difficult/valuable to replace while their power armour became so improperly maintained that they broke more easily against heavier weapons.
After the Heresy ended, the traitor legion loyalists got quietly turned into loyalist chapters and most of the renegades who survived the war returned to the Imperium, proclaimed nominal loyalty and were folded into successor chapters, or were wiped out. Others just vanished into history, perhaps surviving only by virtue of being forgotten by the greater Imperium.
As a curious aside, other Deathwatch fluff says painting over the Chapter badge completely pisses off the machine spirit of the armour. We assume that Blackshields probably get a brand-new set of armor rather than repainting and stripping their existing armor. Or this bit of superstition is just superstition instead of the sometimes-common 40k "you think it's superstition until the laundry machine tries to eat you for insulting it".
Way back during the days of fucking Horus' big temper tantrum, a number of Titan princeps found themselves isolated from their legions, or just plain presumed dead and struck from the records. Unfortunately for them, a Titan isn't a Space Marine, with the result being that they were often believed to be working for the other side and attacked by Loyalist and Traitor forces alike.
The vast majority of the Blackshield Legios were organized along similar lines to their legionary counterparts, being little more than small groups of various different Titan patterns loosely bound together by the need to survive; oftentimes, these groups included members from many different legios and used only one pattern of Titan to avoid logistical issues.
Most of them (like the Legio Tritonis) were opportunistic in behaviour, taking over planets that had been weakened by the traitor-loyalist conflict to create their own pocket empires, or just going full "fuck everything" and raiding planets for supplies without bias or care. Others withdrew to their home worlds and guarded them jealously, blowing up anyone who tried to get too close. Rarest of all (as in, we know of only one case), they were members of a traitor legio who sided with the loyalists; one detachment of the World Eaters-aligned Legio Audax fought their own kind during the Titandeath.
None of them survived past the end of the Heresy for fairly obvious reasons - Titans are neither cheap or easy to maintain (let alone make) without the backing of a larger group like the Mechanicus, and unlike the Blackshields they couldn't just easily vanish into obscurity. Most of them were destroyed by Loyalist forces during the Great Scouring for to trying to break away from Imperial and Mechanicus control alike, or were reintegrated (by force) into the standard Legios.
Brother Vigilant. Being suicidal a Blackshield doesn't exclude you from finding your true calling.
|Forces of the Deathwatch|
|Command:||Forge Master - Keeper - Deathwatch Chaplain |
Watch Captain - Watch Master - Deathwatch Epistolary
|Troops:||Deathwatch First Company Veteran - Deathwatch Champion |
Terminator Squad - Veteran Squad - Kill Team - Kill Marine
|Walkers:||Venerable Dreadnought - Deathwatch Dreadnought|
|Vehicles:||Bike Squad - Rhino - Razorback |
Land Raider (Land Raider Redeemer - Land Raider Crusader)
|Spacecraft:||Kill-Ship - Boarding Torpedo |
Drop Pod - Space Marine Landing Craft
|Allies:||Space Marines - Primaris Marines|