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When watering, make sure the soil is moistened to a depth of at least 6 inches. Melons need extra water during fruit set and development, but too much water during the last week of fruit development will reduce sweetness of the fruit.

These articles will help with growing cantaloupes:
https://www.gardeningknowhow.com/edible/fruits/melons/growing-melons.htm
https://www.gardeningknowhow.com/edible/vegetables/vgen/growing-vegetables-with-seeds.htm
https://www.gardeningknowhow.com/edible/fruits/cantaloupe/growing-cantaloupe.htm

This article will help you:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

I would increase water. The leaves may simply be dying. The weather sounds like it is very dry and hot and the plants are likely going through water rapidly. The black leaves could be leaves that dehydrated quickly.

It may also be a fungus, though not sooty mold if the leaves are not powdery. Treat the plants with a fungicide to be on the safe side.

Yes, it is best to thin your seedlings. When they grow on top of each other, they compete for water, nutrients and other resources and none of them get very strong. For a vining plant like that, I would thin to 1-2 plants for every 6'.

This article will help: https://www.gardeningknowhow.com/edible/fruits/cantaloupe/harvesting-cantaloupe.htm