It seems to me that the point of a pool is to hold water, so you can swim. How do fancy patterns contribute to that?
I wouldn’t have been surprised if dark pools were a trend, like black countertops and appliances. However, my nextdoor neighbors have a pool with a dark gray/black interior, and she told me that last summer, the black surface made the water so warm that it was like a giant hot tub ... not what you want when evening temperatures are in the 90s.
The main selling points are that they are eye-pleasing and they can match theme of your outdoor area or something like that. If you like tiled in-ground pools, you can make your above-ground pool look liket that while you're swimming under water.
Main reason, I think, is to hind the wrinkles in the liner that must better.
I could see black liners being a really, really short fad -- you might have problems telling how far away the side or the bottom was and might find yourself bumping your head if you misjudged.