I am creating this as an issue because the vanilla Nether Brick texture was once lightened in MCPE for the same reason – it was too dark to see individual blocks with reasonable brightness settings. In my case, with app brightness maximized and device brightness set to automatic.
Dark Netherrack is even more dangerous than dark Nether Brick because players can easily fall through unseen gaps and be killed, either by falling damage, lava below or becoming stuck in combat. It rewards players for their purchases with death.
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I know, and that's why I included the vanilla texture for comparison. I'd rather not raise brightness because for one, it washes out everything, causes eye strain and wastes battery, but more importantly, the game should play perfectly fine with the device set to Apple's default (which is the automatic setting) and game brightness at default (50 percent).

Added Nether Brick texture from Fantasy pack. It is even darker and basically black. Even darker than the old (now fixed) vanilla texture.

Are you talking about nether brick? The nether rack lighting looks the same in all your photos.

The files named Fantasy, Natural, and Plastic are all much darker in color than the default texture, the file named Vanilla.
After creating this issue, I found a nether fortress and noticed that it, too, is extremely dark (IMG_0118) compared to the vanilla nether brick texture (not pictured at this point).
This is not a duplicate of 14432. It doesn't even relate to it.
They all seem to be very visible, with clearly patterned textures. The patterns make it significantly easier to detect one block gaps of death. Did you check out how these screenshots look on a different machine? You may want to play with your device brightness settings (e.g. don't set it to automatic).