Dark substrates are definitely better for the fish, as they are more natural. "Dark" is a subjective term, and almost anything other than true white will qualify.
One thing I noticed when I had a pure black substrate was that it was not black but dark grey once it was in the tank wet and under daylight lighting. This rather surprised me, but it shows that the colour of a substrate in air when dry is not likely the same as when it is wet and under tank lighting. Even my playsand which when dry in your hand is a very dark grey (it is a mix of grey, black, tan and white) becomes lighter under water and tank lighting. [So, Akasha, you may now have the "blackest" black you will get...].
The other thing about pure black I found was that every speck of stuff was visible. Bits of plant leaf and such. I never see this with the mixed sand or the previous fine gravel that was also a mix.
If substrate fish are intended--and this is a wide term, meaning not only the obvious like corys and loaches, but substrate feeders such as most cichlids--sand is a better substrate, but make sure it is not rough. JD seems to have found a good sand, and I use play sand which is also ideal. Many filter-type sands or construction sands can be extremely sharp to the fish, and damage mouths and gills.
Byron.