They are a bulletproof species. Can sulk...but recover.
I had a tornado throw my potted tree off a story high deck onto our lawn. The leaves all went yellow. Then fell off...and recovered just fine. Though that was tropical. But these are just tough. A friend had a hardier cultivar and pulled it out put in a cement drive. The drive cracked and up sprang that darn tree.
Black spots make me think possible fungal as well. Unless you've aphids...that can do weird things as well on them for foliage. But I wasn't looking for pest to be honest. As I'm on my phone.
I don't know I would permit a tree showing signs of health issues. To waste energy on blooms. If you are determined to allow it to bloom. Then remove at least half them and don't permit a full on flush. That's how I would handle it.