Painting a Blackberry can be a difficult task, simply because most Blackberrys are made out of color dyed plastic.
Some of the best looking painting cases I have seen were either done with automotive paint or, the best one thus far, is actually covered in gold, by the well known Jacob the Jeweller, as seen here:
One recommendation has been to use Shoe Dye (the spray on kind) as it would prevent the newly acquired color from rubbing off, chipping or scratching easily.
Another option (used in Computer Modifications) would be vinyl dye. You can get it in a liquid for dipping or in a spray can. The dye works by bonding with the surface of the plastic (makes the plastic soft so be very careful) and is very permanent.
Pre colored Cases
ProMobility is now offering pre-coloured cases for 7200 series BlackBerrys as well as select 7100 Series Blackberrys.