I love this!!! I used to refuse buying pots and seedling starters... so we used egg containers, yogurt containers and water bottles. It works really well actually, but I like your idea of using egg shells so that it doesn't rotting!
Keep it up, your garden is awesome. I never seen seeds with that blue on it like your melon seeds... I wonder what that is.
I think it's to protect the seeds and keep their longevity. I don't want to buy too many pots, but I do have some for the continually harvestable plants such as tomatoes, beans, and cucumber. I like to use the 5-Gallon drums for this though. The rest of my containers are anything I haven't thrown away.