The page you're referencing uses data-* attributes and some Javascript to cleverly switch the background image to a blurred one. This explains why you couldn't find the CSS:
<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 needs, you may be able to use Filter - Blur:
-webkit-filter: blur(5px);
-moz-filter: blur(5px);
-o-filter: blur(5px);
-ms-filter: blur(5px);
filter: blur(5px);
If you clearly layout what you're looking for I may be able to help point you in the right direction.
Also, if you don't need the blur effect and just need something semi-transparent the you can use the opacity
(-ms-filter
for old IE) value.