The key signature with only a B-flat is either F major or D minor.
B-flat
F# minor, which has 3 sharps in the key signature.
Piano plays in c.. To transpose to Eb lower the written note one line or space and use the key signature of b and e flat and add the written key signature
In a key signature, one flat is the note B flat.
The key of B flat on the piano has two flats (Bb and Eb).
The key signature for F major has one flat (B flat) , while the key signature for F minor has four flats (B flat, E flat, A flat, and D flat).
The key signature that has B flat, A flat, E flat and D flat is Concert A flat Major.
Assuming the key signature has only one flat (B Flat) then the key is either F Major, or D minor.
A flat lowers the note by a semitone. As a key signature, a single flat indicates the key of F, meaning your B should be flat. Two flats are E and B, meaning the key is therefore B flat. B, E and A flat indicate the key of E flat ... and so on.
There is no key signature that has Bb and C#.
The key signature of the F flat major key has six flats: B flat, E flat, A flat, D flat, G flat, and C flat.
That is the signature for the key of Db. Or, it's relative minor, Bb minor.