Yup, that is one of the reasons the eggs are so dark in color is all the layers of pigment that have to be 'painted' on make for a thicker egg shell. Any change yet, any progress?
Look up the article on assisted hatching and read it. Is there any tapping noise when you hold the egg to your ear? Calling for experts here @WVduckchick, @Pyxis
When you candle the egg, is the air cell about a third of the way down the egg? If you hold the egg up to your ear can you hear a tapping noise? I have had Marans hatch on day 23 but they required assistance. If you hear anything, put the egg back in the bator, goose the humidity again and...
Dark eggs are extremely difficult to candle as you have discovered. However, the fact that you can see the air cell is really all you need. Mark and monitor the progress of the air cell through incubation, if the air cell is growing, usually all is well.