Have you tried to use the filter (for now working only in webkit) ?
Check this: http://davidwalsh.name/css-filters
(I don't have the time to try, but you might have to put two div, one with the image and the filter and another with the form)
Sorry for my english :)
EDIT:
I've made this pen: http://codepen.io/anon/pen/gEmrD
HTML:
<div class="container">
<div class="content">
</div>
<div class="form">
<p>This is you form (with a bit of immagination :P )</p>
</div>
</div>
CSS:
.container {
width: 1000px;
height: 100%;
min-height: 500px;
background-image: url('http://www.gordonviaggi.it/files/wedding_packets/in_giro_per_san_francisco.jpeg');
background-size: 1000px;
background-position: 50% 50%;
display: block;
}
.content {
width: 100%;
height: 300px;
margin: 0;
background-image: url('http://www.gordonviaggi.it/files/wedding_packets/in_giro_per_san_francisco.jpeg');
background-size: 1000px;
background-position: 50% 0;
filter: blur(3px);
-webkit-filter: blur(3px);
display: block;
}
.form {
position: absolute;
top: 200px;
left: 250px;
width: 500px;
height: auto;
background-color: rgba(255,255,255,0.5);
color: #222;
display: block;
}