Instruments haven't been tuned to a particular key in a very long time. Pretty much all instruments are built with equal temperament, meaning the pitches are a

Instruments haven't been tuned to a particular key in a very long time. Pretty much all instruments are built with equal temperament, meaning the pitches are a compromise between what would be otherwise be perfect ratios in different keys. When an instrument is said to be in B-flat (or any other key), this only means that written music is notated in an alternate key so that it is likely to fit on the treble or bass clef without too many ledger lines. It has nothing to do with ratios between pitches or sounding better/worse in that key.