I had 16 of 23 pure Maran eggs for my first ever use of incubator for NYDH, 1 thing I found out after that I wish I'd known at the start--Marans are notorious for taking more than 21 days, up to even 25 days. After upteen billion disasters the only one I had hatch was late in the day of day 23 - but she is fantastic, she never peeped in the egg and went from pip-zip-hatch in total 2 hrs and was up dry & walking faster than any chick I've known, and this am, her day what 4 I believe she actually had a real feather on each wing in the am, and tonight there are 4 on each wing, talk about growing up fast!
I will also caution you - the above egg I candled, or tried to day 7,10,17 , on day 17 my notes says "Very Lg Air cell, likely wide blood ring, no movement, dead" - I *so almost didn't lock that egg down* but now my personal rule going forward is unless it is oozing or smells it goes into lockdown, no matter what, esp. if a dark egg.
Again, I'm an incubator newbie, & I'm clearly learning by trying all the mistakes I can Murphy's Law into, but I really would have had MUCH less stress if I'd known about Maran being classically late to hatch (well used time from what I see in the chick now

Hope this info helps you.