Good Afternoon,
The leaves look like they have an insect infestation such a scales or aphids which like to feed on the sap of magnolia leaves. As they suck the sap from the leaves, they excrete honeydew which drips on the leaves and surfaces underneath. This promotes the growth of blackish sooty mold caused by fungi. These fungi are not plant parasites and do not directly cause plant disease.
The appearance of Magnolia scale or Neolecranium cornouparvum in June, may be due to the warmer temperatures this month.  Magnolia scale is having a good year - MSU Extension
You will see in that article that controlling the scale depends on the stage of the insect. June to early July is the time to apply systemic insecticide which is absorbed by the roots and will move from the roots into the branches.
The next stage in the insect’s life is the active crawler stage which occurs in Michigan in August through September when you can spray with a contact insecticide. You should not need to use the second application if you do the first. Always read and follow the instructions on the label for the best result when applying pesticides.  MSU’s Enviro-weather site has a tool which states when the crawlers are active. 
http://msue.anr.msu.edu/news/magnolia_scale_is_starting_to_appear_on_magnolias 
Finally, if your tree is too large for you to treat, consult a certified arborist. Go to: www.treesaregood.org, the website of the International Society of Arborists to find one in your area.  
MSU Extension’s Plant and Pest Diagnostic Lab can provide a diagnosis of the problem. To learn how to submit a picture/or sample to the lab go to: https://www.canr.msu.edu/pestid/

I Hope this helps.  Thanks for using our service.