Here's the modified HTML structure:

    

    

        

            
        

             a div wih all sort of information
        
    

CSS:

.content {
    float: left;
    width: 100%;
}

.content .overlay {
    background-image: url('images/zwemmen.png');
    height: 501px;
    -webkit-filter: blur(3px);
    -moz-filter: blur(3px);
    -o-filter: blur(3px);
    -ms-filter: blur(3px);
    filter: blur(3px);
    z-index: 0;
}

.opacity {
    background-color: rgba(5, 98, 127, 0.9);
    height: 100%;
    overflow: hidden;
    position: relative;
    z-index: 10;
}

In this setup, the .overlay element has the blur effect applied, while the .opacity element blocks this effect from reaching the child elements, resulting in sharp and focused child elements over a blurred background.