In one of the early drafts of the script they had blue gloves. However, the comic Those Left Behind has the Blue Hands biting it and the Operative being dispatched as a direct result of that.
Even so, the question was about the flashback scene which occurs before the series begins. So they could theoretically have been Blue Hands. Although my understanding (from something behind-the-scenes I read) is that the Blue Hands were supposed to be from some kind of private black-ops organization hired by the Parliament to get River back. That would've come out eventually if the series had gone on.