BorderSide doesn't work

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const CustomContainer(),
    );
  }
}

class CustomContainer extends StatelessWidget {
  const CustomContainer({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Container(
          height: 300,
          width: 350,
          decoration: BoxDecoration(
            color: Colors.blue,
            border: Border.all(
              width: 19, //                   <--- border Work here
            ),
//             border: Border(
//               top: BorderSide(
//                   color: Colors.black, width: 10.0, style: BorderStyle.solid),
//               right: BorderSide(
//                   color: Colors.black, width: 10.0, style: BorderStyle.solid),
//               bottom: BorderSide(
//                   color: Colors.black, width: 10.0, style: BorderStyle.solid),
//               left: BorderSide(
//                   color: Colors.blue, width: 10.0, style: BorderStyle.solid),
//             ),
            borderRadius: const BorderRadius.only(
              topRight: Radius.circular(30),
              bottomRight: Radius.circular(30),
            ),
          ),
        ),
      ),
    );
  }
}