I think most times it’s just a thick shell. Sometimes my birds lay eggs in the mud, and the mud kind of fuses with the bloom, and those are tough to see thru, but oddly, often hatch for me haha. I don’t find my celadon eggs difficult to see into, and I thought I’d have trouble with normal eggs, but not really, unless it has a huge spot over the area I’m looking in. Perhaps it’s brightness of the light? I have a pretty bright candler, but it plugs in, no battery. Perhaps battery ones are often not as bright?
Many people say blue inside eggs are celadon carriers. I had a few blue inside eggs in my original snowies, and I ended up with one blue laying hen in my initial cross of normal egg snowies to my celadons. There may have been more, but I only kept pearls, Italians, and silvers. All birds that I am certain are celadon carriers have blue inside eggs. Some of my carriers lay blue eggs with normal spots. So blue inside and out, but still have spots.