The "Hands of Blue" are a pair of mysterious men, portrayed by Dennis Cockrum and Jeff Ricketts, who wear suits and blue gloves. They are contractors to the Alliance and are in pursuit of River and Simon Tam. They will not hesitate to kill off anyone who has had contact with River, using a handheld device that induces fatal bleeding to anyone near it (except themselves).
The pair are never officially named in either Firefly or Serenity. In "The Train Job" DVD commentary, Joss Whedon refers to them as "the men with blue hands" and "the blue-hands men". IMDb's cast listings name each of them as a "Blue Glove", perhaps the most accurate moniker. However, "Hands of Blue" seems to have become a fanon name for the otherwise unnamed pair, from River's "two by two… hands of blue…" litany (possibly in reference to there being four blue-gloved hands between the two men, or to the fact that the men are merely one pair and may be followed by more like them.)
In the Serenity comics, the blue "gloves" are shown to extend to and cover the upper body as well. They were finally killed and the Alliance passes the assignment to retrieve the Tams to the Operative.