http://codepen.io/jimmythenice/pen/dsxLk
Forked Melindrea’s pen.
Added transition, slightly changed the method. The div is always visible, but is transparent, but the span inside is hidden. On hover, the div is changing background, thus transition works and span is showed.
EDIT: The transition didn’t work, because it needs two states in order to work, like white-transparent and black-semitransparent, like in your example. In Melindrea’s pen there is only one background set and div is hidden when not hovered, so transition couldn’t work.
Have fun!