Flash 8.
symbolInstance.blendMode
Property; a string that specifies the blending mode to be applied to a movie clip symbol. Acceptable values are "normal", "layer", "multiply", "screen", "overlay", "hardlight", "lighten", "darken", "difference", "add", "subtract", "invert", "alpha", and "erase".
The following example sets the blending mode for the first movie clip symbol in the first frame on the first level to add:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].elements[0].blendMode = "add";