I saw your post in the other thread, give people time to respond. Even though there are thousands of readers at any moment, you won't get a good response until someone with experience with Sageretia and experience with black spots shows up. It is appropriate to wait 5 days to a week before complaining that nobody is responding.
I've had Sageretia, found it not to be a very good species in my conditions and have not tried to grow another one in over 20 years. But I am up north, north of Chicago, and Sageretia would necessarily be an indoor tree in my area.
The only black spot I have had any experience with has been on members of the rose family, which includes apples and quince. I also had experience with tar spot on maples in my landscape. Sageretia is related to Buckthorn family, so I don't know which species of fungi could be causing your black spots.
Consider using a rose spray labeled for black spot. If it does not say ''black spot'' as a target disease, on the label it will not work. Follow directions.