It's not possible. Trust me - I have three old blackberry phones laying around that I had hoped to get an alternate OS onto, and I searched high and low to no avail. If the BB had the market share that the iPhone has, perhaps someone, somewhere, would've cracked the bootloader on at least one model, but alas, they're a niche device mostly owned by businessmen, and there's virtually no community of hackers focused on the blackberry.