Blossom end rot symptoms start as water-soaked regions near the blossom end of the fruit in all cucurbits. The affected areas darkens in a widening circle a seen on watermelon here.
The affected tissue becomes dark and sunken and have a dry leathery appearance, and the fruits turn yellow. In case of watermelon this symptom will be similar for natural abortion of the fruit also.
The symptom can spread through the entire fruit at later stages. The fruits may shrivel entirely and collapse. Less water, high water or extreme heat can lead to more blossom end rot symptoms.
In the field, you may also growth of secondary organisms on affected areas indicated by a white fungal growth can grow in the blossom end rot infected area extending upwards.