Update: I went to the car dealer who updated the car's software. Now I can use Bluetooth with my Nokia, after the Lumia Black update. Thx car dealer!
Hope I don't have to recall this message tomorrow ;-)
I also have issues with my Nokia Lumia 925 after the Lumia Black update: when Bluetooth is switched on, my car audio (Volvo C70) cuts in and out. It seems like the radio is being reset every 30 min. which results in no audio for 2 seconds. Very bad!