Can you put some type of an applique over it and then randomly put a few in other places so that it looks like it was supposed to be that way?
My daughter had made a Sun Bonnet Sue with alot of hand applique on it and somehow she got some black oil on it and it wouldn't come out so she hand appliqued a little bluebird over the oil spot and put a few randomly around the quilt so that it didnt look out of place. And no one is the wiser.