Some fruit drop is natural and for a young plant this is a good thing as it encourages new growth. Is the plant putting on new growth? Is there any leaf drop? Some zoomed out pictures might be more helpful.

Some of the newer leaves near the black fruits seem to have possible thrips damage. Perhaps they're affecting the fruit also.

Another possibility could be that your soil mix is staying to wet for too long. The begining of root rot where just the tips of the roots are starting to go, can show up in new growth turning black, including tiny fruits.

But yeah, a pic of the whole tree can help too.

I have a mandarin that has been in the ground for either two or three years (can't remember). Last year, I had the same problem with my baby fruits turning black and falling off. I think it was because it was over watered, plus it may have had a fungal problem so I treated it with Agri-Fos per Patty S. (Hoosierquilt on this board). It still lost all of it's leaves. For fun, I applied fertilizer this past January but the tree still looked horrible. I had decided to just rip it out and chalk up the whole thing to a failed experiment. But the day my husband and I were going to rip it out of the ground (we literally had a shovel to dig it up), I noticed little tiny leaf buds. Now we're in April, and I religiously test the soil to see how the moisture level is doing. The tree has lots of healthy green leaves and about 5 or 6 baby fruits. Not a lot, but the important thing are the healthy leaves, which it has never had before.

So this was a very long story, and to sum up... my 2 cents is that it's too wet! Lol! Good luck to you!

I have same problem with my potted mandarin. Lots of fruits turning black and hard. But some survived and grow bigger now. I will monitor the watering using 3-way thermometer that can tell us if it is dry or too wet. Thanks for trail of conversation above.

Wierd, I hope it goes away for you