这个效果可以通过CSS Pseudo-Classes实现如下，不需要JavaScript！
如下所示：

body,main::before {
  background: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/80625/tree.jpg) 0 / cover fixed;
}

main {
  margin: 100px auto;
  position: relative;
  padding: 10px 5px;
  background: hsla(0,0%,100%,.3);
  font-size: 20px;
  font-family: 'Lora',serif;
  line-height: 1.5;
  border-radius: 10px;
  width: 60%;
  Box-shadow: 5px 3px 30px black;
  overflow: hidden;
}

main::before {
  content: '';
  margin: -35px;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  filter: blur(20px);
  z-index: -1;
}

<main>
  <blockquote>"The more often we see the things around us &#45; even the beautiful and wonderful things &#45; the more they become invisible to us. That is why we often take for granted the beauty of this world: the flowers,the trees,the birds,the clouds &#45;
    even those we love. Because we see things so often,we see them less and less."
    <footer>&mdash;
      <cite>
        Joseph B. Wirthlin
      </cite>
    </footer>
  </blockquote>
</main>

在Codepen：https://codepen.io/jonitrythall/pen/GJQBOp上可以看到实例

快速注意：

根据您已经设置的网站结构，您的Z-Index可能与示例中描述的不同。