Hatching dark eggs can be very iffy, and a lot of people experience difficulty with them, particularly when the eggs were shipped.
I sand my eggs when they are shipped, especially when they are VERY dark.
I only sand until I'm just through the color, and then I mist the eggs with a betadine-water dilution the color of iced tea. This covers the contamination that could occur now that I've removed the bloom.
The idea is to promote evaporation to allow the chick to grow smaller than it would if it contained all of the moisture from the start. Chicks that develop too large due to high humidity can't turn correctly. They are also prone to drowning if evaporation doesn't occur as well as it ought to.
I have had too many chicks large enough that they couldn't pip or hatch properly- fully formed chicks that didn't make it at hatch. Once I started using this method (got it from an Emu farmer!) I have had significantly better success with shipped eggs. It doesn't seem to really be necessary with local eggs (my own) and I'm not certain why. When hatching in a really nice incubator, like a Sportsman or an rcom, it also doesn't appear to be necessary, but with most affordable models, it's a helpful method. This may be due to the higher flow fans in the nicer models noted.
I do increase humidity at the end of my hatch, as per my Incubation Cheater, the same for Marans as for other eggs. My feeling is that if you have evaporated the proper amount during the first 18 days, you can't drown them with humidity at the end. Humidity merely prevents evaporation, it doesn't make eggs 'take-on' water.
This works well for me, but like most on BYC, I don't consider myself an egg-spert... I just like to share my egg-speriences to help anyone else who might be able to learn from them!
Sanding the Marans, Penedescenca, Welsummer eggs
1. Use an emery board or 400+ grain sandpaper. I use a circular motion and make sure I'm moving the hand with the sander, not the egg. Try not to rattle the egg unnecessarily. Sand in fine lines removing 25%-30% of the color, only just to the white part, not deeply.
2. Put the eggs on a paper towel and mist them with a 30% povidone iodine to 70% water blend.
3. Wait for it to dry, turn, repeat, dry, turn, repeat. You can't be too careful about making sure the whole thing gets lightly misted, but don't