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