JavaScript (ES6), 108 bytes

-2 bytes thanks to @l4m2

s=>[..."000055007500"].map(n=>a[(+n+a.indexOf(s))%12],a="C#DD#EFF#GG#AA#BCDbDEbEFGbGAbABb".match(/.[b#]?/g))

Try it online!

JavaScript (ES6), 106 bytes

-4 bytes thanks to @l4m2

Expects a string in [A-G][#b]? format. Returns a list of strings in the same format.

s=>[..."000055007500"].map(n=>a[+n+a.indexOf(s)%12],a="C#DD#EFF#GG#AA#BCDbDEbEFGbGAbABb".match(/.[b#]?/g))

Try it online!

JavaScript (ES6), 110 bytes

s=>[..."000055007500"].map(n=>a[(+n+a.indexOf(s))%12],a="C#DD#EFF#GG#AA#BCDbDEbEFGbGAbABbBC".match(/.[b#]?/g))

Try it online!

JavaScript (ES6), 108 bytes

-2 bytes thanks to @l4m2

s=>[..."000055007500"].map(n=>a[(+n+a.indexOf(s))%12],a="C#DD#EFF#GG#AA#BCDbDEbEFGbGAbABb".match(/.[b#]?/g))

Try it online!

JavaScript (ES6), 90 bytes

Expects a note in [A-G]b? format.

s=>[..."000055007500"].map(n=>a[(+n+a.indexOf(s))%12],a="CDbDEbEFGbGAbABbB".match(/.b?/g))

Try it online!

JavaScript (ES6), 110 bytes

s=>[..."000055007500"].map(n=>a[(+n+a.indexOf(s))%12],a="C#DD#EFF#GG#AA#BCDbDEbEFGbGAbABbBC".match(/.[b#]?/g))

Try it online!