I have a Grandmother's Flower Garden hand made by my grandmother in the 1920's. I the dark spots problem and was really worried it was mildew. Had it appraised and was told that the bats in the early part of century were likely either cotton or wool and the cotton was not needle punched or had resin in it so needed to be closely quilted. And that my dark spots were pieces of cotton seed and not to worry--helped date the quilt as authentic.