Perhaps it's just my Northeastern prejudices, but to me, black suits don't connote
intelligence agencies as much as they do
law enforcement. The difference is subtle: in my mind's eye, the guys in the
uniforms defer to the short, bullish fellow in the
black American- or British-style suit, who tends to be verbally (as opposed to physically) abusive and bossy -- think
G. Gordon Liddy with an attitude. He may or may not wear
sunglasses, and if so, they're completely dark or mirrored.
Secret Service and other
Feds also wear these suits, but they're marginally cooler.
It starts getting creepy, however, when the man in the Grey Suit comes in (either flannel or linen), especially with military tailoring. He's slightly feline in movements, and may appear not to be physically threatening. While other fellows are openly armed, he may not appear to have a weapon at all. Don't be fooled. He can most probably stop you dead in hand-to-hand from across the room, and can draw a semiautomatic out of thin air. While the Uniforms tend to be physical, the Black Suit is verbal, the Grey Suit hardly speaks at all. Sometimes he'll gesture with his pen, or speak (in a hushed, not whispery tone) a few words. If he must speak with you, it's in a cultivated, professional, yet vaguely friendly tone that suggests he's heard a great deal about you. He's probably not wearing sunglasses, but you'll never quite remember what he looked like afterward.
And then, it starts getting weird....