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This can be due to a great number of things. Has it been excessively wet? This can cause an infection which will do this. Wettable sulfur can help stop and prevent future infection.

Cold can do this as well. If it has been chilly in your area, then it will resolve itself when the weather warms up.

It could also be a deficiency if you don't fertilize at least once per year.

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These articles will help you.

https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-houseplant-care.htm
I'm including this link, because it show the pit potted.
https://www.gardeningknowhow.com/special/children/sprouting-avocado-pits-how-to-root-an-avocado-seed.htm

This tree will not be harmed, as long as there is plenty of nutrients in the soil. Just put soil over that spot. I do notice that the soil is overly wet, though. I can tell by the beneficial fungus that is taking over the top of the soil. The fungus is not hurting the tree, but it is a signal that there is a significant amount of dead matter that is being "eaten" by them.

Make sure to let the soil dry out almost completely on top before watering. As long as there is even a little moisture as far down as 2 inches, then it is still plenty wet.

I advise correcting the watering issue, or that healthy tree will soon suffer a very drastic change in appearance, as they are susceptible to a plethora of diseases.

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https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-fertilizer-tips.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-pests-and-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/pruning-avocado-trees.htm

When fertilizing potted trees, make sure you feed correctly and do not over apply. Excess fertilizer can build up in the soils over time and can harm the plants.
Blackening leaves is a sign of salts high in soils. Watering with conditioned water also can cause this. Distilled water or water that sets out for 48 hours will work.

Are these from last year? You can remove them to make room for fresh fruit to appear.

They do tend to like potting soil amended with compost, and dolomitic lime in container. It is very important to give them as much light as possible, which will mean supplementing with 200 watts or more of horticultural grade lighting indoors, and through the winter until it can go outside to get proper lighting.

This article will give you more information on this: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm