Would the vassals and smallfolk under the Brackens and Blackwoods share their liege lords’ rivalry due to border raids?

Good question! If I had to guess, there’s probably two simultaneous phenomena going on. 

For the folks who are definitively on one side of the border or the other, you’re going to get mirroring of the feud, because those smallfolk see their liege lords as “their” lords, their protectors, and the other side as the thieving, murdering bastards who keep raiding their lands. Indeed, a lot of these people are going to be the folks who the lords turn to first to make up their feudal levies, they’re going to have been involved in a lot of the fighting and raiding, so there’s a strong element of selective hypocrisy here, similar to how the border reivers from the 13th through the 17th centuries had their clan feuds on both sides of the Anglo-Scottish border, despite the fact that these clans were basically indistinguishable.

For the folks who are in the middle, things are going to be more complicated. Since the land shifts back and forth so much between Bracken and Blackwood, you’re not going to get clear-cut divisions of loyalty. Rather, there’s likely to be a lot of mixed loyalties, both inside families and between generations. Here, I’m reminded somewhat of a lot of the German peasants during the Thirty Years War who made sure to own a picture of both the Pope and Martin Luther and then swap them on the wall depending on which marauding army was passing through.