It'd be nice if XenForo had some sort of indicator to show that users blocked for toxic entries are blacklisted by hand, not necessarily evidence submitted recently by forums. While these bans are normally put in place for obvious reasons, sometimes they are outdated. It can be confusing if a certain email hostname always requires admin approval for registration, but there aren't any reports matching that hostname in the SFS database.
There's not much documentation about this feature on the SFS website. After I asked about an unusual case on their forums, a moderator told me the special meaning of 255. The owner confirmed that this is the expected behavior.
For example, here's what I might see now in "Users Awaiting Approval":
StopForumSpam matched (email: 255)
[code]StopForumSpam matched (email: blacklisted)