That's why competition is important in the market for security. Without competition, a single supplier becomes a monopsony, which is as bad as a monopoly.
Competition makes everything better. Choices improve quality.
I don't believe it's exactly a monopsony, for the simple reason that there are only two major parties here: The U.S. DoS (the buyer, or contractor) and Blackwater (the seller). It's neither monopoly nor monopsony when the only buyer chooses to a) only deal with one major seller and then b) literally give all effective power and management of resources to the seller, and grant all of the seller's requests for constant additional resources from the buyer. There is no effective/total control of options from either end. It's just called stupid.