Hi Edward, thanks for the question.

What you are seeing on your magnolia is magnolia scale.  This is a large-scale insect that seems to be part of the tree.  The black is the honeydew that are excreted by the insect. Flies, butterflies, wasps like the sugary substance.  It is a serious pest.  If possible, remove heavily infected branches and treat the tree.  Both systemics and contact insecticides can be used but timing is critical.  Systemics used just after flower drop will reduce the chances of harming pollinators.  Contact insecticides should be applied when crawlers are active in the August/September timeframe.  A degree day model for when the crawlers are active, can be found here:

Spongy Moth, Black Vine Weevil, Cottony Maple Scale, and Magnolia Scale Scouting - Deciduous Plants - MSU - Enviroweather

Check out the following links for more detailed information:

Magnolia scale is having a good year - MSU Extension

E-29.pdf (purdue.edu)

Home, Yard & Garden Newsletter at the University of Illinois

Magnolia Scale | Horticulture and Home Pest News (iastate.edu)

I hope this helps.