The black stuff in sugar is often charred sugar particles that occur during the sugar making process.
The black stuff or black specks in sugar can also be burnt sugar or caramelized sugar.
If the dryer process is not complete when making the sugar, caramelized sugar can escape into the sugar.
It doesn't mean the sugar is bad and it's still safe to use.
Black stuff can also be bugs such as ants or flour weevils although most often it's just caramelized or burnt sugar.
Sugar does not get moldy because sugar is actually resistant to microbial growth.
Sugar is a stable substance which does not harbor any bacteria or mold like other foods can.
The lack of moisture in sugar also deters the microbial growth.
Sugar actually has an indefinite shelf life and can last for decades.
Although sugar can become hard but it remains safe to use for years and you can break the hard sugar up and it will also dissolve in liquids.
While the sugar itself does not grow mold the sugar bag when kept in damp locations can grow mold.
Granulated sugars can also become harden or lumpy and sugars in honey and syrup can also crystalize.
Even when the sugar has become lumpy or crystalized it still is safe to eat consume the sugar.
The best way to store sugar is to store the sugar in a cool, dry airtight container in an odor free location.
Store the sugar in a cool, dry place such as the pantry and away from any extreme temperatures and sources of chemicals and use a container with a tight fitting lid to prevent bugs from getting into the sugar and clumping.