Well if you like the blue then you could try and swing the service before you buy it. Pretty poor IMHO to sell a car that is not really ready for you to drive away with it. Depends where its being sold tho, I got mine and BMW did the service 2 for me for free because it was close and I said I wouldn't buy the car without it.
Problem with BMW paint is that its quite soft and the black cars are renowned for swirls and scratches. My previous car was a carbon black and it was bad for it but looked fantastic in the light. My grey Z doesn't show any marks at all but grey isnt for everyone, it doesn't shine in the same way but I fancied a change.
Cragswinter has just bought a Z4Mc in Ruby Black, gives a really nice red glow
http://www.z4-forum.com/forum/viewtopic.php?f=2&t=34497