Most Military helicopters are usually painted very dark grey or khaki, so they, like Cats, will tend to look black at night.
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The good thing about helicopters is the abillity to fly low and circle, police/news/military does this all the time.
The bad thing is that they're very very noisy, and if, for example, the local Police one round these parts use it (or used to, they don't seem to do so quite so much lately, perhaps to save Money) they loiter in a sinister fashion all day, watching through their electronic surveillance devices, it can be a bit intimidating, it certainly can.