My penedesencas are from welp hatchery and they are flighty. It looks like they are crossed with welsummer anyway. I will be getting rid of them after I collect a few eggs for an olive egger project with my wheaton ameraucana. I plan on getting partridge and crele eggs from a breeder in the spring.
Marans, especially Black copper marans (BCM) usually will lay the darkest eggs. Overall are a mellow bird. Cons- are big so they eat a lot. Often go broody after the first year.
Welsummers, pros- seem to be hardy, nice average size and feed consumption, average friendliness. Cons- not as dark eggs as marans more of a terracotta flower pot color eggs
Barnevelder, pros- awesome patterned birds, friendly cons- not DARK dark egged, not the best feed efficiency
Pendesencas, pros- great consistantly dark eggs, great feed efficiency, free range well cons- flighty, big combs that freeze, can be harder to find a source for (hint get eggs not chicks)
Empordanesa These I have no experience with but they seem like a white pendesenca?
These are the only dark eggers I know of.
Aprille
I have had one FBCM (French black copper Marans) hen that was just super-duper sweet and friendly and would hop up on DHs shoulder and hang out with him while he worked in the yard and what-not. Her eggs were REALLY dark. Shortly after reaching POL, she was beaten up by a light brahma and never recovered from her injuries & died about 3 weeks later after only laying about a dozen eggs.
I have one girl that is a cross between a penedesenca, FBCM, and blue ameraucana... the breeder was going for an olive egger, but some of the offspring grew up to lay dark brown rather than olive eggs, and my Olivia is one such girl. Her eggs are about the color of a milk chocolate candy bar.