Dec 05, 2005 - Flex2 PopUp Window Transparency
In Flex2 the TitleWindow defaults to having a transparent title bar

To make the transparency of the titlebar in the TitleWindow / PopUp window set the panelAlpha attribute from 0 to 1. 0 being fully transparent, and 1 being fully opaque (so if you wanted 50% alpha put in 0.5).

<mx:TitleWindow xmlns:mx="http://www.macromedia.com/2005/mxml"
panelAlpha="1">