instanceNameOfTextarea.backgroundColor = 0x000000;

\\\\\\\\\\\\\\\

To make the background area of a TextArea component transparent, then you will want to use the following ActionScript.

_global.styles.TextArea.backgroundColor = undefined;

by Jen deHaan

还有这样可以使背景透明...
不过只能100％透明，不能调节透明度

\\\\\\\\\\\\\\\\\

用这个就连边界也消失透明了：

InstanceNameOfTextArea.depthChild0._alpha = 0;