Since Vangelis (at the time of the original cinema release) was unwilling to release his music, it prompted an orchestral recording by the "New American Orchestra" for an album release. Two tracks did, however, show up on Vangelis' compilation album Themes, in 1989, which included Blade Runner's End Credits and Love Theme from Bladerunner.
In late 1993, a bootleg appeared by a company calling themselves Off-World Music (OWM) which, aside from containing almost the entire Vangelis score, also contained the Ladd Company Fanfare by John Williams, Harps of the Ancient Temples by Gail Laughton, If I Didn't Care by the Inkspots (which was used in early workprints), and the audio portion of the Blade Runner movie trailer. The sound is fairly clean sounding, though not on par with the new official Vangelis release that came out in mid-1994. It also includes a nice 8-page booklet with detailed liner notes.
The rumour that Vangelis' official album release was prompted by the appearance of the bootleg is disputed by Vangelis associate Andrew Hoy in Future Noir: The Making of Blade Runner by Paul M. Sammon where Hoy states that "Vangelis wasn't even aware that there *was* a bootleg until after his official soundtrack came out."
Sometime in 1995/96 a supposed Romanian bootleg appeared under the Gongo label, which contained most of the material of the OWM bootleg and added the Japanese vocal song broadcast by the blimp. However, it excludes the Inkspots song and the trailer audio and contains no liner notes. Track titles are listed in Romanian. Although it claims to be produced there, I personally suspect it came from elsewhere.
As to why did Vangelis "really" take this long to release his album? Common rumour has it that it was due to Ridley Scott's constant re-cutting of the film, which frustrated Vangelis, who was forced to continually re-adapt his music. The OWM liners state that it was because of contractual disputes, since Ridley "edited the score and supplemented source music for the final print." Future Noir rejects both theories, but is strangely vague about any *real* explanation, leaving it as a "Topic for Future Research".
The main differences between the two boots is that the OWM contains "If I Didn't Care" by the Inkspots, which was eventually replaced by "One More Kiss, Dear" and a track which contains the audio portion of a Blade Runner trailer. The Gongo CD contains a Japanese vocal which is broadcast by the blimp in the film. The OWM release also includes a detailed 8-page booklet, with a track-by-track breakdown. Also given the fact that everything is in English, I think the OWM is preferable over the Gongo on all counts. Unless of course you absolutely *must* have the blimp song. Then you must get all three. :)
However, since the OWM is extremely hard to come by, the Gongo release makes for a cheaper alternative if you can find it. Last I heard the Gongo was going for around $40.
Of course, the cheapest alternative of all is simply the official release, which actually makes for a nice listen, IMHO. Of course, if you want just the music, you'll have to resort to the boots.
For sound samples from both official and unofficial Bladerunner soundtrack albums then go direct to to
http://bau2.uibk.ac.at/perki/films/brunner/br.html
Thanks to whoever posted this originally. Unfortunately I've lost your details so if you recognise your work please let me know!