Try placing a dry cloth over the shiny mark then lay a damp cloth on top and press lightly with a hot iron . Allow the blouse to cool, then brush the collar gently with an unwanted or old clean toothbrush - this should reduce the shine a bit.
If it doesn't work (depends on the fabric - a synthetic material will have melted unfortunately), then you might be able to buy an adhesive or sew-on *applique* to hide the mark. Another solution would be to buy some black lacey fabric, cut out a *collar shaped* piece and sew it to the collar.