Add the transition properties to the element itself rather than the :hover
pseudo-class version.
In doing so, the transition will take place when hovering on and off.
Updated Example
.img-blur {
transition: all 0.35s ease-in-out;
}
.img-blur:hover {
-moz-filter: blur(4px);
-webkit-filter: blur(4px);
filter: blur(4px);
}
<img src="http://i.imgur.com/Vp5StNs.png" class="img-blur" />
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