Any day now I’ll plant out my tomatoes – they are outside and weathering the cold nights, I’ve not had a problem with blossom end rot for a few years but, I don’t know if that’s because I give them ground up egg shells or not but for what it’s worth here is the process.
Blossom end rot is caused by lack of calcium, egg shells can be put on the compost heap but take years to break down so I always dry mine and add them to a jar, I have used them to deter slugs but I find my slugs seem to have a rhinoceros like foot and just plough over them. But to use them to add calcium to the soil I add them when plant out in the hole and over the top before I fill in. Just scatter a handful of crunched up shell.
Of course all the egg shell in the world won’t do any good if the plant doesn’t have enough water to take it up- do don’t let them dry out!