That makes perfect sense.
With the progression:
G → Bm → Am7
V ? vi
the Bm is acting primarily as a linear/passing chord. Notice the bass line:
G → B → A
and the chord tones:
G = G B D
Bm = B D F#
Am7 = A C E G
The notes B and D are common to G and Bm, so the Bm sounds like a smooth extension of the G harmony before settling into Am7.
Another way to hear it is as a temporary iii chord in G major:
G major scale:
G A B C D E F#
iii = Bm
ii = Am
So for a moment the harmony leans toward G-major colors before returning to the song's overall key center.
What's especially interesting is that you've already identified several "color" chords:
Cm
D♭maj7
Fm6
Bm
This is very characteristic of sophisticated Motown and R&B writing. Rather than staying strictly in C major, the song moves freely between:
- C major harmony
- borrowed chords from C minor (Cm, Fm6)
- chromatic color chords (D♭maj7)
- smooth voice-leading chords (Bm)
If I saw G → Bm → Am7 in a chart, I would probably think:
"The Bm is there because it sounds beautiful between G and Am7."
The voice leading is more important than assigning it an elaborate Roman numeral.
You can think of it as:
G → Bm → Am7
V vii/?? vi
but in practice I'd simply call it a chromatic mediant/passing chord connecting G and Am7.
That's one of the things that makes songs like All I Do Is Think of You so rich—the harmony is driven by elegant voice leading and color, not just traditional functional harmony.
