Some of the leaves on my violet plant tend to rot or turn dark brown and wither and die. I have read articles about leaves turning yellow, but not turning black and rotting. I water my plant once a week and fertilize about every other week.
This is definitely a fungal infection. You can dilute peroxide with an equal part water whn you water next, and this will help get rid of the problem. It is a chance that the soil is already suffering from a bad infection that might not be able to be repaired if it is already showing symptoms on the plant, then it can be hard to get rid of at this stage, but peroxide will be your best bet. Here is an article to refresh you on their care: https://www.gardeningknowhow.com/houseplants/african-violet/african-violet-care.htm