Here the effect of applying it: http://i.imgur.com/3lPjH.png
with the following setting in my color-theme[1]:

--8<---------------cut here---------------start------------->8---
(org-block-begin-line ((t (:underline "light grey" :foreground "#008ED1" :background "#EAEAFF"))))
(org-block-end-line ((t (:overline "light grey" :foreground "#008ED1" :background "#EAEAFF"))))
--8<---------------cut here---------------end--------------->8---

The only thing that you *should* change in my patch is the color of the
background you want, by default, in a dark theme.

Is the above working for you, guys?

Best regards,
  Seb

Footnotes:
[1] It should work without any customization. But, of course, if I did "need"
    2 new faces, it is well to customize them...
-- 
Sébastien Vauban