You need to make sure that the BitmapData object is initialized with transparent background-fill-color (4th parameter for the constructor). This should work (if you have text in TextField):
var snapshot:BitmapData = new BitmapData(textField.width, textField.height, true, 0x00000000); snapshot.draw(textField, new Matrix());
See also as3 docs for the BitmapData constructor
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