If you still have to deal with Flash Action Script 2 TextArea, this my be useful.
Transparent background:
_global.styles.TextArea.backgroundColor = undefined;
This one Works globally on all TextArea components, if you need background on some of them use:
myTextArea.setStyle("backgroundColor", "0xFFFFFF");
No focus highlight:
myTextArea.drawFocus = false;
No border:
myTextArea.setStyle("borderStyle", "none");