If this is incorrect, I sincerely apologize for my lack of research, but I have not been able to find the key of B-flat minor (b-moll) in any of Bach's works other than the Well-Tempered Clavier Book 1 and Book 2. It might exist somewhere, but in any case, it seems certain that it is an extremely rare key.

And there is no doubt that B-flat minor is the key that expresses the most sorrow among the keys Bach used. Its expression is even stronger than F minor. In this prelude, for example, in measure 4, beat 3, the tenor and bass clash with a minor second, and in measure 17, beat 1, the soprano and alto likewise clash with a minor second. This is a compositional technique to express sorrow.

The theme gradually ascends as B-C-D-flat, C-D-flat-E-flat, D-flat-E-flat-F, depicting the swelling of sorrow.

In measure 13, there is a momentary calm, but from measures 15-16, the drama of sorrow reaches its peak again.

And the weighty chord on beat 3 of measure 22 serves as the cadence.

The entire piece is dramatic, revealing an unexpected side of Bach. Performers should never suppress their emotions but express them fully.