<div class="blurBg">
  モザイクもざいく
</div>

.blurBg{
  width: 100%;
  height: 350px;
  text-align: center;
  font-size: 32px;
  line-height: 350px;
  background: url(https://cruw.co.jp/wp-content/themes/understrap-child-ver2/images/top/web_marketing.jpg) no-repeat center;
  background-size: cover;
  position: relative;
  z-index: 0;
  overflow: hidden;
}
.blurBg:before{
  content: '';
  background: inherit;
  -webkit-filter: blur(5px);
  -moz-filter: blur(5px);
  -o-filter: blur(5px);
  -ms-filter: blur(5px);
  filter: blur(5px);
  position: absolute;
  top: -5px;
  left: -5px;
  right: -5px;
  bottom: -5px;
  z-index: -1;
}