Another way is to use a simple wrapper around FileReader returning Observable:

  function toBase64(blob: Blob): Observable<string> {
    const reader = new FileReader();
    reader.readAsDataURL(blob);
    return fromEvent(reader, 'load')
      .pipe(map(() => (reader.result as string).split(',')[1]));
  }

Usage:

toBase64(blob).subscribe(base64 => console.log(base64));

Another way is to use a simple wrapper around FileReader returning Observable (snippet is in TypeScript):

  function toBase64(blob: Blob): Observable<string> {
    const reader = new FileReader();
    reader.readAsDataURL(blob);
    return fromEvent(reader, 'load')
      .pipe(map(() => (reader.result as string).split(',')[1]));
  }

Usage:

toBase64(blob).subscribe(base64 => console.log(base64));