Nope. A letter of is a granting of permission to a private citizen, by our government, to seize the property of another nation, usually on the high seas. When used properly, it is also almost exclusively issued to redress a
specific grievance by that private citizen against the nation in question.
That is NOT the function Blackwater was performing in Iraq, and if our government claims that their power to grant such letters allows them to hire others to do the job of our military forces then that is just further evidence of their total usurpation of the Constitution.