A Blackshield was a Space Marine who fought in the Age of Darkness with his Legion's heraldry struck from his armour, his allegiance and origin deliberately unknown. In an age when brother slew brother and loyalty could not be read from a badge, some warriors blacked out their plate and cast off their names, becoming warbands answerable to no primarch and no cause but their own.
Their reasons were as varied as their number. Some were loyalists cut off from any chain of command, survivors of the Shattered Legions who fought on without hope of orders. Others were deserters sick of the war, renegades who had turned on both sides, or fugitives fleeing crimes committed in the war's chaos. A Blackshield might be a hero or a monster, and often there was no way to tell which until the killing began.
The phenomenon captured the moral collapse of the age. When the great certainties of the Great Crusade shattered, thousands of Astartes found themselves adrift, their oaths meaningless and their masters unworthy. To go Blackshield was to admit that the old world was gone, and to face the war on one's own terms in a galaxy where no banner could any longer be trusted.