I should have phrased this better. There have always been mercenaries of course, and I believe there has been an uptick in US government use after the end of the cold war under Clinton. I'm however under the impression that the growth since 2001 has been unprecedented, as is the fact that the number of private sector employees deployed to war zones by the US government outstripped military deployments. So a qualitative change seems to have taken place during that time, even if it was just the continuation of an earlier development.

I could easily be wrong though, because I'm having trouble finding precise and dependable information online – if you have some good pointers about the growth and changed role in US politics of mercenaries, I'd be interested in reading them.

I have no numbers, but I'd agree the use of contractor forces has expanded. However, it clearly was an emerging trend already during the Clinton era, and even before that.

Excellent point. I wonder if those who equate Blackwater with "mercenary sector" would also equate U.S. Navy’s Military Sealift Command with "mercenary sector" as well?

I don't know. Is the U.S. Navy's Military Sealift Command a private entity that sends armed employees or contractors to war zones to fulfil military roles?