Just stole the papers for the horse Frost.

I did kill some black briars though to get it...does this effect any of my thieves guild missions do you know?

as far as know, it should not. Maven is essential and i am pretty sure she is the only black-briar in the thieves guild questline. could be wrong though.

Black-Briars? You mean the mercenary guards? No effect on anything. I don't even recall any dialog noting your methods whether you slaughtered everyone or were completely undetected.

All the Black-Briars are essential I believe. Unless you removed all their flags via console, for science, in which case we look forward to your results.

You'll be fine. Maven wants you to steal them.