Are there any guidelines for appropriate use of the Black HUD UI in OS X?

Does anyone know what the HIG are saying about the black HUD UI in OS X?

From stackoverflow

• See the Transparent Panels section of Apple's Human Interface Guidelines.

  Graham Lee : best. conversation. ever. "What does document X say about Y?" "Read document X."

  splicer : LOL... I figured he was looking for the relevant section of the HIG (a HUGE document).

  Graham Lee : HUGE - but searchable (I was only referring to it yesterday) ;-)

• Basically HIG say that black HUD UI is named a transparent panel.

  Its role is the same as a standard panel, i.e., an auxiliary window that offers some controls related to the current document.

  Its look is as discrete as possible, in particular in highly visual applications.