Look up maple tar spot to see if it might be that. It could also be something else, of course.

The poor fall color would be due to conditions the tree has experienced this season, perhaps next year it will do better - this introduction is known for coloring well despite unsuitable conditions.

I bought a 'Red Rage' nyssa this fall. It is still in the pot and got dried out a couple times since I bought it, causing it to droop (but not shrivel or burn). The leaves turned yellow and gradually fell off, from the bottom up.

I went back to the nursery and the rest of the 'Red Rage' still there are still in leaf, with a few of them starting to turn red. So apparently the drying out caused mine to behave out-of-character this time. Presumably next year, after having been in the ground for months it will turn red, later in the season.