Needs CalMag, no joke.
"The causes for squash end rot are simple. Squash blossom end rot 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, there isn’t enough to sufficiently build the cells on the fruit. In particular, the bottom of the fruit, which grows the fastest, doesn’t get enough calcium. As the fruit gets larger, the cells begin to collapse, starting with the weakest cells at the bottom. At the location of the squash blossom, rot sets in and a black indentation appears. While the causes for squash end rot will not make the squash dangerous to eat, the lack of calcium frequently causes the fruit to mature too early and the squash will not taste very good.
Read more at Gardening Know How: Squash Rotting On End: Squash Blossom End Rot Causes And Treatment
https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm "