Those Welsummer eggs are awesome!
I have Marans (Cuckoos), they can lay a nice dark egg--but the genetics are tricky. Breeders are always working to not only get a darker color, but consistent color. That's the real trick--all eggs hatched from one breeding group aren't always going to produce hens that lay that nice dark egg. And the egg color also changes throughout the laying cycle--so, the first eggs laid will be the darkest--as the season progresses, the eggs will lighten, sometimes considerably. After a period of non-laying (such as a molt) the first eggs will again be dark.
Hatchery Marans aren't (typically) going to give you a correct Marans egg. There is a color chart (see here: http://marans-club.club.fr/echllang.htm#echelle ). If a hen does not consistently lay a #4 egg or above, it cannot by the standard be called a Marans regardless of its breeding.
Also, Marans eggs are often more of a reddish-brown. Here's a pick of my Marans eggs(this was taken a few days ago, so these are late-season eggs; they will be darker in the spring):
I would think that if you want to breed and want a challenge, Marans are a good choice. If you want a breed with an egg color that is consistent, I'd go with the Welsummers.
I've heard that Penedesencas lay a very dark egg, I think even darker than Marans or Welsummers. But many don't like to keep them because of their very flighty temperaments. I have no experience with them, but I've been told that they are, er, totally nuts! A breeder I know says he has to keep them in a very quiet, enclosed place and try not to startle them--he says he's actually had birds kill themselves by slamming into walls just because he walked up to their pens.
And I thought my Polish were goofy!