Hi.
I raise French black and blue copper Marans. They are great birds, but I agree they are FAR from "rare". Sadly the only thing that makes a bird rare is whether or not you are willing to spend a little more than average on chicks.
Check out this link for LOT"S of good information...
http://www.maransofamericaclub.com/black-copper-marans.html
Marans eggs aren't tricky to incubate... maybe to candle though. I use "dry" incubation... keeping it around 20-30% humidity through day 18 and then 60-65% for hatch. I sell Silkies and Marans as fast as I can hatch them... for $9/ straight run chick. I sell my layers for the same value as ANY laying bird. Somewhere above that 9 up to $30 once they are laying.
Note that hens get their eggs shade from their sire and cockerels from their dames... being sure you have a darker laying rooster will make all the difference in your flock. Be sure to grow them out past 16 or 20 weeks because I have white feathers pop up now and then in the tail region on males. You wanna cull for things like comb sprigs and split wing.
And for the love of God... feeding high quality formulated ration makes a HUGE difference in your hatch-ability and the viability of chicks! These are dual purpose birds... don't raise them like layers. The need about 18% protein to do really well. And 22% protein was shown to give the best hatch rates. Keep low nutrient snacks like scratch to a minimum. It is empty calories without the added vitamins, minerals, and (importantly) amino acids... that ARE in formulated rations. I choose to feed Purina Flock Raiser with 20% protein and oyster shell on the side for layers. This works well and is a happy medium for my whole flock including roosters, chicks, juveniles, layers, broody's and so on. It is readily available at a price I can afford and usually has a recent mill date. $3 cheaper per bag at one store so shopping around can be worth while.
My eggs are around a 5 with occasional darker than that (and lighter). I am satisfied to maintain. Though SUPER dark is pretty... I'm sure it diminishes the amount of eggs laid.
The things my eye catches and my selection choices... have changed since year one as I become familiar with the breed. All the reading and information is nice... but experience really brings it all together.
The Marans are also PLENTY broody. Only laying 3 eggs or so per week... I don't do breeding pairs. I like to keep about 4-5 hens and I practice line breeding... done correctly you never have to worry about bringing in "new blood" or disease.
If selling is your goal... know your market.
And know that getting a good breeding program going takes a lot more time than I would have thought, and it isn't uncommon for me to be a season behind my goals.
Also... running black/blue/splash together dilutes your black and blue to me, so I prefer keeping my colors separate if I'm doing more than one. And crossing a blue egger to a Marans does NOT equal a true Olive egger as many like to claim on their first generation crosses.
That's mom's egg in the middle and one of my BCM pullets on the top. Pretty good olive color, no? Why would crossing a blue egg layer and a Marans not deliver an olive egger? 
Wish I could get them!
Hoping you find them a home!
