I live in Everett WA. I’m planning to start my tomato seeds in the house in March as I did last year, and move them out in May. Last year they were somewhat successful. A few actually ripened normally either in the garden or after I brought them in. But many more rotted before ripening, or had big brown spots. This was very disappointing, and seemed to get worse as the season wore on. Some I salvaged by cutting around the bad spots or picking and using green. But I would have had 3-4 times the tomatoes if this hadn’t happened. I have a plot in a p-patch that is organic gardening only, which doesnt bother me. I would do that anyway, probably. Somebody said that they sell “blight-resistant” seeds, but none if the packages Ive looked at say that. I do plan to get those wire cones to keep the plants upright this year. I think that a few other people had this problem, from what I could see, but none as bad as mine.
Try spraying the plants with Garret Juice.
Your tomato problem sounds like blossom end rot. Spray the plants with Magnesium Sulfate (Epsom Salts)