Here are some of our split to BS boys.
In my limited experience, the split BS boys have a greater or lesser degree of this dark coloring on the upper wing, and it possibly increases over time.
Usually split to blackshoulder peacocks will have more black on their wing than what is normal. It helps if you are very used to seeing the wings of a regular India Blue peacock, which you I am sure are used to seeing with Colbalt. You will notice that on a peacock split to blackshoulder, he will have black going up into areas where the barred wing feathers are normally placed.
Here is a photo I grabbed off of google (specifically from bamboo4u) as an example of a split to blackshoulder. This is an extreme example and very obvious since he has lots of black, but also barring.
Another example off of flickr - this one with a bit less black:
A more subtle example from someone on BYC:
The funny thing is the first two pictures I found just searching "India Blue Peacock". I think as split to blackshoulder peacocks get older, the extra black area gets larger. It would be interesting if split to blackshoulder peahens show rust colored flight feathers. I am not sure if they do though.
This first picture is different of the others !
This is probably a IB black shoulder split barred wings ( if it exist ! ) .... because his pants is black !