'93ish. The fully welded all aluminum frame came out in 1992, but I'm thinking that was the Lime green colored model.
1990-1991 was the Alien ACX, aluminum/cro-moly combo frame. The full aluminum frame was much stronger than the combo frame, which was prone to cracking and breaking, especially the rear triangle at the dropouts. I had two BITD, sold one, the other slowly disintegrated over time. Bought a third frame a few years ago, put all my left over original components on it. It's strictly for asphalt use now, as the frame does have some cracks in the seatpost. I've chosen to delay fixing them, because I would like to leave the paint original for as long as possible.