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The B Diminished Triad, B Diminished 7th Chord, and B♭7th Chord
In the above example, progressing from F major to the B diminished triad or the B diminished 7th chord (instead of B♭7th) would mean moving the chord root six seimitones. Let's look at the notes in each chord:
B diminished (Bº, VIIº) triad, the notes are: B D F
B diminished 7th (Bº7, VIIº7), the notes are: B D F A♭
B♭seventh (B♭7, ♭VII7), the notes are B♭ D F A♭
The progression IV – VIIº is the only progression in the circular harmonic scale where there are six semitones between the root notes of adjacent chords, instead of five semitones (Table 47).
TABLE 47 Semitones Between Chord Roots in the Harmonic Scale
So, if F7 were to progress to B♭7, then B♭7 would be the secondary dominant of E♭. So B♭7 would function as a pivot chord, taking the progression into the new key (key of E♭).
However, F7 can also proceed pretty smoothly to Bº7, which is harmonically close to B♭7:
Bº7 = B, D, F, A♭
B♭7 = B♭, D, F, A♭
And, being highly unstable, Bº7 seeks to move on to the next chord, which is E7. So the progression remains in the prevailing key.