A lot, but the depends on quality if work. To give you an idea, I had my wing and door repaired and resprayed to a HiGH standard and that alone was £200.......youll probably be looking at upwards a grand and over.
a lot of good body shops wont let you self prep... i enquired about a cinq full respray at a reputable place round 'ere and nearly fell over when he said £2000. although for the quality of work it would have been worth every last penny.
look into vinyl wrap... if its done right you will never be able to tell
I second what Rich said. You'll be paying a least half the value of the car to get it done, and to change from only blue to black it's not much of a change for the money - you could sell yours and buy a black one and have money left over. There's quite a few wrapping places round my bit that do a very good job - you can't tell the difference at all. They do full wraps from £700 but it depends on the size of your car and quality of the material used. The places round here add a 3 year guarantee as well.