Clearly they hobby in a way that differs you. And there is nothing wrong with that.
This is the main 'mission statement' for any part of the hobby, and for dark AoS or Inq28, even more so . So what if someone's creativity is limited too primary colours, or using stock minis.
If everyone painted and posted nothing but John Blanche inspired (copied) stuff, it would become repetitive and boring. Blanchitsu has its place, but so does every other artist or modeller that has inspired to move away from 'mainstream' AoS or to create warhammer art over the years. Perhaps someone is just starting out, and is not used to painting in the dark grimey style, or perhaps they just cant paint like that. Or, most shocking, maybe just doesn't want to paint like that.
Instead of moaning about how other people choose to interpret what is dark (which can be very subjective) or what is 'allowed' to be posted, how about add some positivity. Offer advice to those that ask of it, or even choose to ignore those that post things that dont mesh with your vision of the hobby. If everyone stopped trying to class whether work should be defined as ' AoS28 or Blanchitsu' or not, we have a lot more people showing off cool minis, and lot less people refusing to post here or the facebook group, for fear of being berated for being 'creatively wrong'. Nothing wrong with guiding the style, but the constant negativity on the facebook group is turning people away, to the point they are complaining about it in other groups.