11-08-2011, 11:51 AM
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A photo or two would help to confirm, but from your description I would surmise the "black" is not the sand itself but the accumulation of organics. This is normal and in fact wanted. Fish waste and other organic matter will settle on the substrate and work its way down into it. Bacteria then break down the organics into nutrients. Malaysian livebearing snails are helpful in this process, and so are any snails; these break down the larger bits into smaller particles that the bacteria can more easily handle. MLS also burrow throughout the sand, adding further benefit.
Another point is the colour of the sand. If you have white or very light coloured sand, you will likely see this more easily. With darker sand it is next to invisible. Substrate fish also factor into this, and you mention corys; they help by eating any bits of food that would otherwise fungus and add to the organics.
My comments are assuming what you are seeing is comparable to the build-up of dark gray/black matter along the front glass, just below the surface of the substrate.
Keeping the organics down is important; not overfeeding, and a fish load balanced to the aquarium (water volume and plants) is needed.
Another possibility might be very dark green slime, called cyanobacteria. If organics are high, this can occur, often among floating plants closer to the light source but it is also capable of covering parts of the substrate. This film would be easy to dislodge as a sheet.