change style on click react

Html
<div id="app"></div>

Css
button{
  width: 80px;
  height: 40px;
  margin: 15px;
}
.blackButton{
  background-color: black;
  color: white;
}

.whiteButton{
  background-color: white;
  color: black;
}

React
class Test extends React.Component {
  constructor(){
         super();

         this.state = {
              black: true
         }
    }

    changeColor(){
        this.setState({black: !this.state.black})
    }

    render(){
        let btn_class = this.state.black ? "blackButton" : "whiteButton";

        return (
             <div>
                 <button className={btn_class}
                         onClick={this.changeColor.bind(this)}>
                           Button
                  </button>
             </div>
        )
    }
}

ReactDOM.render(<Test />, document.querySelector("#app"))