I was thinking the standard military's special forces - although certainly some Blackwater people are former special forces. That's essentially what the ninjas were: soldiers for hire that specialized in infiltration with small teams.

I think the "specialized in infiltration with small teams" is the more important part than "for hire", which is why I thought of the standard military's special forces, and not modern-day mercenaries. Feudal Japan's customs made the "for hire" part necessary, but modern day customs do not.