Like tomatoes, peppers and squash, watermelons can also be affected by blossom end rot. This condition happens due to a calcium deficiency. Calcium helps a plant create a stable structure. If a plant gets too little calcium while the fruit is developing, the end result is what you're experiencing. There are a few things you can try for blossom end rot treatment.
Water evenly, not too much or too little. (https://www.gardeningknowhow.com/edible/fruits/watermelon/watering-watermelon.htm ) Add a low nitrogen fertilizer to the soil for optimal calcium uptake. Add lime to balance the soil’s pH if it is too low, (should be between 6.0 and 6.5). Add some gypsum to the soil for additional calcium. In addition, remove affected fruit and use a calcium rich foliar spray on the plant.