The chord progressions for the song Nomadic Revery by Bonnie Prince Billy are:
Verse: (C Fm Bb Fm Bb C)x2 Cmaj7
Chorus: (Bb C Fm Bb C)x2
Bridge: (Cm Eb)x3 Gm (Cm Eb)x3 Bb Cm (Cm Eb)x3 Fm Gm
The Bridge seems to clearly be in Cm, but I can't understand what key the verse and chorus are in. Are they in Cm, but change temporarily to F/Dm back and forth during the verse? Or is it possible to use the major version of the minor tonic chord for effect? Thanks.
You will not find a scale that contains all those chords. But you don't have to. There is a term "chromatic". It describes notes and chords that don't use the notes of the "scale of the key". You don't need to "borrow" chords from anywhere, it's perfectly alright to be firmly in C major but use a Fm chord. Think the end of that very simple tune "When The Saints Come Marching In". In C, we have these chords C, C7, F, Fm, C, G7, C. That's not a string of modulations, merely a sprinkling of chromatic chords in C major.
And yes, it's very common to swap minor and major versions of the tonic chord.
Listening to "Nomadic Revery" on YouTube, it seems clear that both verse and chorus come "home" to C major. The bridge goes on a bit of a wander, but I wouldn't change the key signature.
(What a truly terrible song, and equally terrible performance! But each to his own...)
Apart from the common use of the major and minor chords that occur in songs - I,IV,V,ii,iii,vi - another set of chords is sometimes used, from the PARALLEL key. Thus, say in C, the chord pool is C,Dm,Em,F,G,Am, but also Cm,Eb,Fm,Gm,Ab,Bb. I've left out the more rarely used dims.
For the Verse: C Fm Bb Fm Bb C
It´s clear that C is the tonic, but as you said early the rest of the chords seems to be taken from Cm intead of C. That´s a vague answer so let´s examinate it, shall we?
In C: I iv bVII iv bVII I
I iv it´s also V i, so let´s see what happends in terms of Fm.
In Fm: V i IV i IV V
This is now easier to look at, because we´re talking about the Melodic Minor scale (i ii +bIII IV V vi° vii°).
Despite the name, the scale has a Dorian sound with the difference of V instead of v; that´s why someone above recomended to use F Dorian.
But as we discussed already the tonic is C not Fm. Being C the fifth of F, that means the tonality used for the verse it´s the fifth mode from F Melodic Minor, also known as "Hindu scale", so the final answer is C Hindu (I ii° iii° iv v +bVI bVII).
Even thou we came to a satisfactory conclusion it is equality as right to use C Minor, F Dorian, Bb Lydian Dominant, etc.. (As long as you avoid dissonances like C against Cm). In practice whatever suits you it´s cool as long you and your partners can understand with each other.
Actually Melodic Minor is a scale used together with regular Minor scale (also called Natural Minor for no confussion). Knowing these we can arguee how it moves from C Minor to C Hindu, because it may be interpretate as if it goes from Cm to Fm.
Now about the Chorus: Bb C Fm Bb C. We have the exact same chords as before so it should be the same as before.
The key to undertand this kind of complexier harmony is first: find out if there´s any cadence (V I; VII I; IV I) at the end of the progression, that should tell you were is the tonic; next the second is to look at what kind of relationships you have compared with the tonic, if you know the modal chords (II #iv° vii lydian; I v bVII mixolydian; i ii IV v vi° Dorian; i ii° v minor; i bII v° bvii Phrygian; i° bII biii bV bvii Locrian), then you can find what the progression "tastes like", for example here I saw "i IV" and I was "yup, Dorian sauce" but in Dorian the fifth is v not V, so work wasn´t over yet. Searching scales with the Dorian flavour (1 b3 5 6) I got Melodic Minor which again, despite the name it is really a Dorian scale.
Look: Dorian (1 2 b3 4 5 6 b7); Melodic Minor (1 2 b3 4 5 6 7); Natural Minor (1 2 b3 4 5 b6 b7) as you see Dorian and Melodic they are almost identical if weren´t by that b7 against 7. A better name could have been Dorian Leading Tone or maybe Harmonic Dorian, but the name is part of history now.
Probably a bit to extense and late, but I hope this helps anyone who reads it.
As far as I know, there are no key signatures that contains such combination of chords. It may be an accidental piece. It is there to add flavor to the song. You can revise a song and add some diminished chords.
Disclaimer: I haven't heard the song.
Considering the use of the Cmaj7, among other reasons, I'd probably call this a C major. It has some borrowed chords from its parallel minor, C minor. Additionally, in jazz, Fm7-B♭7-C would be the (very common) backdoor progression, with C major as the tonic.
