The webpage in question utilizes data-* attributes and Javascript functionality to cleverly alter the background image to a blurred version. This is why traditional CSS styling couldn't achieve the effect:
<div class="image-full-bleed is-usingCanvas" data-scroll="blur-cover" data-image-url="https://d262ilb51hltx0.cloudfront.net/fit/c/1200/960/gradv/29/81/60/darken/25/1*fDLP8pfTZX3cAQB0J6a72g.jpeg" data-image-blur-url="https://d262ilb51hltx0.cloudfront.net/fit/c/1200/960/gradv/29/81/40/darken/50/blur/50/1*fDLP8pfTZX3cAQB0J6a72g.jpeg"><div class="image-src picker-target" style="background-image: url('https://d262ilb51hltx0.cloudfront.net/fit/c/1200/960/gradv/29/81/60/darken/25/1*fDLP8pfTZX3cAQB0J6a72g.jpeg')"></div></div>
Depending on your requirements, you might find success using the Filter property - particularly the Blur function:
-webkit-filter: blur(5px);
-moz-filter: blur(5px);
-o-filter: blur(5px);
-ms-filter: blur(5px);
filter: blur(5px);
If you provide more details on what you're seeking, I can offer guidance in the right direction.
Additionally, if you don't require the blur effect and simply want something semi-transparent, you can utilize the opacity
property (-ms-filter
for older versions of Internet Explorer).