blur of part of Background image with css

If it has to be dynamic, you should have some trouble, but you can have somewhere to start with this :

HTML

<div class="background"></div>
<div class="mask">
    <div class="bluredBackground"></div>
</div>
<div class="content"></div>

CSS

.content {
    width: 70%;
    height: 70%;
    border:2px solid;
    border-radius:20px;
    position: fixed;
    top: 15%;
    left: 15%;
    z-index:10;
    background-color: rgba(168, 235, 255, 0.2);
}
.background {
    width:100%;
    height:100%;
    background-image:url('http://www.travel.com.hk/region/time_95.jpg');
    z-index:2;
    position:fixed;
}
.bluredBackground {
    width:100%;
    height:100%;
    display:block;
    background-image:url('http://www.travel.com.hk/region/time_95.jpg');
    z-index:1;
    position:absolute;
    top:-20%;
    left:-20%;
    padding-left:20%;
    padding-top:20%;
    -webkit-filter: blur(2px);
}
.mask {
    width: 70%;
    height: 70%;
    border:2px solid;
    border-radius:20px;
    position: fixed;
    top: 15%;
    left: 15%;
    z-index:10;
    overflow:hidden;
}

FIDDLE