piano scale
B♭ altered
The seventh mode of melodic minor. Every degree above the tonic that can be altered, is.
The same sounds spelled from the other side are A♯ altered.
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
What is true of it
B♭ altered is B♭, C♭, D♭, E♭♭, F♭, G♭ and A♭, and back to B♭.
Its steps are H – W – H – W – W – W – W, where W is a whole step and H a half.
Written correctly it needs 8 alterations, E♭♭ among them. That is what the letters force, and it is also why nobody writes music in this key: the same notes spelled from the other side are easier to read.
On the same tonic, B♭ Locrian differs from it by exactly one note.
Chords made only of these notes
Found by testing every chord in the catalogue against these 7 sounds, not chosen.