Certified GKH Gardening Expert
First I do not recommend raw, uncomposted chicken manure. This can require the proper bacteria in the soil to break it down into a usable form of nitrogen. Otherwise, you will be stuck with urea, which will burn your plants, and cause a number of issues, including pH, fluctuation. This ia what I believe has happened here.
When the soil pH swings, certain thing like calcium and magnesium become unavailable to the plant. This causes what's known as blossom end rot. Which is what you are experiencing.
My treatment is as follows: apply dolomitic lime to the soil area. This will correct pH, and supplement both calcium and magnesium.
Here is an article with more information: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm