I came to this site to ask the SAME question. I plant yellow squash and zuchinni every year and this has never happened. My plants look like something is cutting them off right below the blossom. My plants bloom and then drop off! I use a similar soil mixture and water daily. So anyone with any help... that would be great. All my plants are dying and fast. How do you fix this? Can you fix it? Also what causes it?
Any help would be appreciated.

Poor supply of Ca. and slower rate of transpiration cause blossom end rot.Water the plants to keep the soil wet.Dry soil around the plant will slow down the transpiration.Feed the Ca. by foliar spray.

Blossom end rot is from a lack of calcium in the soil. I had it last year after a very wet spring. I got organic bonemeal (sold by Miraclegro got it at Lowes) and mixed it in to the top layer of the soil with water, cleared right up! Now if only my early blight this year was as easy to get rid of....