It’s a myth that ground eggshells prevent blossom end rot. Egg shells decompose too slowly to be effective as a calcium fertilizer but they are still a welcome addition to compost as organic matter.
If you want a calcium boost faster than slowly decomposing eggs shells, first crush them into a fine powder. I use the blender, you can also use a pestle and mortar.
Then squeeze a lemon onto the crushed egg shells. This makes calcium citrate. Mix it one part to ten parts of water then water it onto the soil near the base of the plant.