Yes, I had a discolouration of the skin around the ostomy site. I'm not sure if it's exactly what you describe; mine was sort of an after-impression of the barrier. You could actually see the square outline of the flange on the skin for quite some time. The skin there was darker and somewhat shinier than the skin on the rest of my body. Even my surgeon commented on this. But I had my ostomy for almost 2.5 years, though, probably why I had marks left.
The discolouration eventually faded gradually, but it took quite a while as I recall, many months in total. I did use bio oil and cocoa/shea butter to try to smooth it out.
I agree that skin reactions like this are likely dependant on our individual skin type/complexion. My skin is pretty pale and generally sensitive. To be honest I was surprised I wasn't allergic to the barrier!
I also think skin changes probably become more apparent the longer you have the ostomy. With that in mind, perhaps more people don't post much on the topic because most people getting a j-pouch don't have ostomies for more than a few months, maybe not long enough to cause an issue like this (I was definitely an exception). Also, perhaps it's because something like this is more of a cosmetic concern than a medical issue, so it doesn't make its way to the forum. But, if your skin does not hurt or itch and there is no real irritation, I wouldn't worry excessively. Most likely your marks will fade or at least get lighter with time. I suppose you could always ask for a dermatology referral if it's really bothering your or doesn't seem to be improving.