Noymira - everyone is a little different in what they choose to hatch. If you want dark egg laying offspring, choose the darkest eggs. If you have particular traits you are trying to get rid of in your flock (slow growth, late POL, egg eating, sparse shanks etc., then hatch only the dark eggs from the hens that don't have those negative traits. I've hatched little pullet eggs from my BC and they hatch and grow out really nicely, with better hatching rate than the big 80 gram eggs I've tried to hatch.
Hudsonnascarfan - I think it depends on the hen alot. I've had some start laying mediums from the beginning, and they get bigger after a few weeks. It also depends on when they reach POL, I've found that in many of my hens, of different breeds, the earlier they reach POL, the longer they lay pullet eggs, and the more gradually they get bigger. I have a hen who started laying at 17 weeks, around mid-May, and her eggs are still smaller than my wheaten hen hatched at the same time, who didn't lay until 26 weeks. I still haven't gotten a non-doubleyolker from either of them that I would consider a large egg. Give them time. My one hen has been laying for about 18 weeks now, and her eggs are still getting bigger.
On a different note: Lots of talk about traits in the past few posts that refer to "wheaten influence". I'm still skeptical that pale shanks are wheaten/BC hybrids, as all of the BC that I have seen here in Europe have pinkish shanks with some dark scales. Even the pictures on the French Club website show very pale shanks. I have two 9 month old roosters from German lines that have very pale shanks, but are otherwise properly colored BC. I plan to cross one of them to a wheaten hen later this fall to see if he throws any wheaten chicks. IMHO I think that what many folks often refer to as "wheaten influences" are just the genetic enigmas that mystify most BC breeding programs. It's been discussed at great length in many posts, that pairing up two outstanding birds still produces mossy, over melanized, bare shanks, straw hackle etc. I don't think it's as often that someone still has wheaten blood running in their flock.
And as for the pale shanks, I'll try to have an answer for that later this fall. I promise pictures and everything. But really my guys shanks are really pale, but not as pale as their penmate wheaten roo's shanks. Oh, and I don't think straw hackle comes from wheaten either - My wheaten rooster looks almost identical to my BC roos, except for his wing triangle, and no black stripes at all in his hackle. Color-wise, hackle and saddle in my BC and Wheaten roos are very close. This may change though after their next molt... we'll see.