The reason why Joe Zawinul used an inverted keyboard.
Black Market, Weather Report
You can read an interview and know his very vague answer about this subject.
He was playing rif melody and ad-libpart with his right hand. Anyway, I imagine his concept of playing on those reversed keys. This is only my pure imagination. But if a good saxophonist like Wayne Shorter plays sopranosaxophone always biside a jazz pianist, it is very normal that he would have great desire to play sopranosaxophone himself. Maybe Joe Zawinul couldn’t help getting a sopranosaxophone. He bought a sopranosaxophone somewhere and played pentatonics in his house, in a closet. And he made the melody of Black Market. He wanted play Black Market as a Weather Report repertory but he couldn’t play his sopranosaxophone in the presence of Wayne Shorter.
Leaning to left side, he is imagining playing his sopranosaxophone.
That is the reason why Joe Zawinul used an inverted keyboard and why Wayne Shorter didn’t play the theme with his sopranosaxophone and why there was a very long tenor saxophone solo, and why Black Market is in Bb major.