Never actually caught a jet black myself. But Bedhapmpton Waterworks, just to the north east of Portsmouth along the rail track there used to be fairly heaving with them, last time I passed that way. I also saw an individual one in some woodlands edge around about Basingstoke or somewhere like that. Doing a bit of work up there and as we pulled onto the A road, this one appeared directly opposite the motor.
Can't really say as I've ever personally encountered the other colours people speak of. Though a mate reckons he's had blue ones from the outskirts of Pompey too. On the north western