I think these guys use SoS to sneak up on players. On a crawl up the middle to the lava area we got jumped 3x by this mob, even when I had a player watching behind us. Ironically they seem to backstab one player and then move on to another, giving the group a chance to react. With the BS damage they're capable of, if they stayed on one group member they might be able to waste him before anyone could react, especially in a boxing situation. As it is they hit very hard and make for a challenging fight.
After spending some time doing this camp I've revised my theory. They do NOT use SoS. What they do is they path a little closer to the walls than the other kinds of Scarlet Legion mobs and perhaps they have a slightly bigger aggro range.
This means that the walls do have safe spots but you have to be scrunched up right against the wall or the blackguard will detect you. This can be a problem with a group since people tend to slide out from the wall during the course of a long camp. I keep my casters scrunched up tight and leave the warrior slightly forward so IF the blackguard aggro's at least he attacks the right toon.