Как такое можно реализовать на react.js?
state = {
  block1: false,
  block2: false,
}

componentDidMount() {
  setTimeout(() => this.setState({ block1: true }), 3000);
  setTimeout(() => this.setState({ block2: true }), 6000);
}

render() {
  const { block1, block2 } = this.state;
  return (
    ...
      {block1 && <div class="block-1">Блок 1</div>}
      {block2 && <div class="block-2">Блок 2</div>}
    ...
  );
}
UPD. https://jsfiddle.net/jaLgf9w7/
state = {
  block1: false,
  block2: false,
}

componentDidMount() {
  setTimeout(() => this.setState({ block1: true }), 3000);
  setTimeout(() => this.setState({ block2: true }), 6000);
}

render() {
  const { block1, block2 } = this.state;
  return (
    ...
      {block1 && <div class="block-1">Блок 1</div>}
      {block2 && <div class="block-2">Блок 2</div>}
    ...
  );
}
UPD. https://jsfiddle.net/jaLgf9w7/