The New York Times today briefly attempts to clarify the murky issue of Blackwater's immunity deal with the State Department. According to the Times, Blackwater was offered a special type of immunity called a Garrity Warning, typically offered to law enforcement officials who may have done something illegal. Technically, the Garrity Warning, seen here, does state "[a]nything I say may be used against me in any subsequent department charges." But as the Times points out, "this form of limited immunity does not bar a criminal prosecution but is seldom granted in a case in which a criminal prosecution is likely."
P.S. The original Garrity case
involved police officers. As private contractors, it seems somewhat
dubious that Blackwater employees would even be eligible for a Garrity
Warning. Any lawyers out there who know more about this are welcome to
elaborate in the comments section.