Basically, blackshields were not formal during the Horus Heresy. During that time period, however, 'orphans' from various legions struck out for themselves or stayed loyal to the Emperor. In this instance, I am going with that because a loyalist marauder warband would allow for a variety of characters thrust into a chaotic situation. I also feel like a small group of Space Marines can make a hell of a fun roleplay, even with the culture clash.

To clarify, I think that there are bound to be Space Marines merely cut off from their Loyalist Legions, perhaps survivors of a betrayal by another legion element that massacred their own. They might well fight under their own colors but not have a command to fight under. Odds are, they're happy to just have other loyalist Astartes by their side in a fight. Others will be real blackshields; removed livery, renounced their traitorous Primarch, fighting for the Emperor.