Just getting into a DW campaign with a few friends, and I know that black shields are the brothers that request to remain anonymous to the rest of their squad mates for some reason. My question is, does a black shield have to be a marine from the loyalist chapters, or could they be perhaps a redeemed traitor legionare?

They can be from Traitor Legions, but bear in mind most Black Shields are watched very closely for signs of corruption or sedition.

That much I gathered, but good to know that being a redeemed traitor legionare is possible. Currently, the group consists of an imperial fist tactical marine, a Templar assault marine, dark angels apothecary, and a (rather absent minded) blood angels librarian. I was thinking why not throw myself into the mix as a Night Lords black shield, should lead to some fun confrontations throughout the course of this campaign.

Not to sound like I know your crew, but from the way I've seen Black Templar's usually roll "fun confrontation" comes out more like "killed to death because of heresy" if you're origins are ever found out.

That said it could be fun to occasionally run into things that might revel you to the rest of your Kill-team or eventually have one or maybe two of the others know and both have that as a threat to use against you while at the same time trying to keep the knowledge from spreading to anyone who might feel like instantly and blindly killing you a lot.

That's where my role as the groups marksman comes into play, that sniper rifle is a nasty piece of equipment for anyone to deal with, and the GM has already hinted at some stealth/subterfuge missions coming up, and as a 40k Templar player....I know just how unsubtle they can be haha.