Your title said scale on magnolia but you said scale on hydrangea in the body of the question. I am going with the magnolia. There has been a lot of magnolia scale this year. Scale will not go away but will only get worse until it kills your tree. Scale are live insects that reproduce. We have a very informative publication on scale. Click on the drop down boxes and read it thoroughly. The black you are seeing on plants below the tree is mold. Scale secrete honeydew (poop) which is a sticky syrup. It attracts other insects but also will become moldy. You can try spraying your plants with a strong spray of water to remove that. You do need to treat your tree, however, or take it down as the scale will eventually kill it.  https://extension.umn.edu/yard-and-garden-insects/scale-insects-minnesota-trees-and-shrubs