Obsidian, nether bricks and other blocks with similar colors are almost impossible to distinguish. It's cool shader overall, don't even need to tweak a lot of settings to make it look good (except for mapping, waving and emissions), but, unfortunately, it's impossible to use until I can see all the blocks properly.

It improved a bit in a latest update

Before:

After:

Good enough I would say. I guess it's not easy to make dark blocks brighter without ruining other blocks.