Hi Barbara,

Thank you for your question.  40-50 feet tall sounds like a mature tree and I know you are concerned.  

First we need to determine if it is a tree that sheds its bark as it grows.  I'm guessing that is not the case because you would have noticed this in years past.  

Next, without photos, I am at a disadvantage on knowing the tree type and what the black spots look like.  

So, trees can loose their bark due to stressful situations in the weather (too hot/too cold), animal damage, declining health.  Thinking about the rest of the tree, do the leaves look normal, was there recent changes around the roots (compacted with heavy machinery, change in soil level, water changes?)

If the rest of the tree seems healthy, you might want to get an arborist to take a look at it and give you a better idea.  Here is a link to a similar questioning answer that was submitted https://ask2.extension.org/kb/faq.php?id=831907 click it an see if this helps.

Good luck with your tree