How old are they?
If I were you, unless it is significant, I would keep all of them for the time being. I would mark them via a leg band and make a note of what you are seeing in a record book. Reason being, they will go through several juvenile molts and you may find that what you are seeing improves as they develop their adult plumage. Or, it could become worse. Growing them all out also gives you the opportunity to see how the line you are working with develops. In my opinion it is always good to learn by experience, although kind of expensive when it comes to feed. Haha!
Some refer to what you are seeing as barring, others call it ticking, and others call it stress bars that result from feather growth, feed or conditioning. A few others think it is the result of an unknown modifier gene. I think it is most correctly termed “fretting,” not to be confused with “fraying” of the feathers. Maybe in time scientific research will give us a definitive answer.
Here are a few articles on the subject of fretting. I always read info found on the internet with a bit of apprehension unless it is the result of an actual scientific study. Unfortunately there is a lot of conflicting info out there! Selecting away from it is a good bet and a logical solution.
https://www.themodernapprentice.com/fretmarks.htmhttp://www.sabong.net.ph/forum/showthread.php?76961-Feather-Fret-Marks-and-lines-on-feathersI raise all of my self blue birds under the exact same conditions and see variances of what you are seeing in different birds. Some have none of what you are seeing present, others have a very minimal amount, and others a moderate amount. I automatically cull for a moderate amount. My birds are very spoiled and I am sure yours as well, so I don’t necessarily think it occurs due to a nutritional deficiency. I raise our self blues on an endless supply of high quality feed and supplement them with healthy treats like cabbage. Perhaps it is a weakened portion of the feather that occurs during growth/development. I like to add a dollop of Red Cell to the waterers of our breeders for extra vitamins and nutrients, and I think I will do that with the self blue grow outs as well next spring to see if it makes any difference.
I don't believe it to be actual barring, as barring is referred to in the standard as (1) transverse markings of two distinct colors and regular in shape, (2) alternate colored crosswise markings caused by a recessive autosomal factor, or (3) defective coloration represented by transverse purple markings in blade feathers.
The American Poultry Journal, Volume 20 gives details regarding (3). I'll see if I can post a link. The Book of Poultry by McGrew also does. Oldies but goodies....some of the info within them may be outdated.
https://books.google.com/books?id=ENNIAAAAYAAJ&pg=PA364&lpg=PA364&dq=transverse+purple+barring+poultry&source=bl&ots=MzqSQmKp2Z&sig=ACfU3U1_X1Xm_bquQ_C1YXeZz9T1vf5xEw&hl=en&sa=X&ved=2ahUKEwjOw4Xy4YLkAhXGJt8KHQXLBnUQ6AEwA3oECAkQAQ#v=onepage&q&f=falseIn (my) experience, it seems to occur more frequently with the use of blacks or splits. It could be that it is just how I am hitting the odds each season, but more birds out of my splits have it present than birds out of my self blue x self blue breeding pens. Self blue x Self blue typically produces the most consistent, uniform color. However, blacks are important for setting better type and also for ensuring the self blue cockerels are properly melanized. Perhaps some of the members with more breeding experience than me can chime in with their experience and knowledge regarding what you are seeing.
As the saying goes, you have to build the barn before you can paint it. When the grow outs develop their adult plumage, select and keep the ones with the best type and most uniform self blue color. There are no perfect birds out there. If you don't own the American Poultry Association SOP book, I highly recommend it as it goes into detail on defects and disqualifications. The proposed standard for self blue is on the ABC website, and there are many birds in the gallery section that display proper type. I am a visual learner, lol!
There are a host of other things to watch for and keep in mind with self blues, such as type, feather quality, combs, head width, body width, vigor, egg color etc so I would keep them all and then narrow down to the very best, culling for disqualifications along the way. Good luck on your program and keep us posted on how it comes along!
Here is an interesting study on how the self blue plumage is believed to be created.
https://bmcgenet.biomedcentral.com/articles/10.1186/1471-2156-9-7