piano scale
B♭ Phrygian
B♭1C♭♭2D♭♭3E♭4F5G♭♭6A♭♭7
Minor with a flattened second — the half step right above the tonic is the whole character.
The same sounds spelled from the other side are A♯ Phrygian.
Written out
Every accidental is written in, rather than left to a key signature, so the letters can be read one at a time.
The step pattern
B♭HC♭WD♭WE♭WFHG♭WA♭WB♭
What is true of it
B♭ Phrygian is B♭, C♭, D♭, E♭, F, G♭ and A♭, and back to B♭.
Its steps are H – W – W – W – H – W – W, where W is a whole step and H a half.
It costs 6 alterations to write — near the limit of what a key signature can carry.
On the same tonic, B♭ natural minor differs from it by exactly one note.
Chords made only of these notes
D♭majorD♭7dominant seventhE♭mminorE♭m7minor seventhFdimdiminishedFm7♭5half-diminished seventhG♭majorG♭maj7major seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.