The darkest will be the Black Copper Marans. The "good" lines of these birds will lay a chocolate brown egg. They are hard to find. I have a roo and 3 hens that I hope to start laying this month then I have 3 teenagers and I hope that I am picking up 10 more next week. I will be offering eggs for sale and spring should be about right. send me a pm if intrested for the details.
The grocery store probably sells sex link eggs. That's my guess.
Wellsummers lay beautiful dark eggs, often with speckles. Cuckoo Marans lay a pretty dark egg. Some breeders might sell you chicks (10 or 15 minimum) if you don't have an incubator. Newnanchick is correct that Copper Black Marans lay perhaps the darkest egg, but they are hard to get and very expensive. My experience is that cuckoos can ay as dark an egg as the Copper Blacks.