Silt Blocks are a type of soil that are very similar to Sand Blocks that start appearing from the underground surface (very rarely). Silt is affected by gravity and will deal damage to any player or enemy they fall on. Unlike sand, it can stop the Hallow, the Crimson, and the Corruption from spreading. It looks similar to Ash Blocks, but is a lighter color.
When the Wall of Flesh is defeated for the first time, and the "Spirits of Light and Darkness" are unleashed, Silt Blocks inside the new Corruption, Crimson, and Hallow biomes will be converted to Ebonsand/Crimsand and Pearlsand respectively, much like regular sand. After this initial conversion, however, Silt Blocks are not affected by the spread of Corruption and Hallow.
Just like Slush and other forms of sand, it disappears when it falls on a rail track.
Using the paint sprayer on a falling silt block will not affect it.
Silt can be used in the same way as Sand to fill lava pits. It can also be used in creating traps, since it causes damage to whoever or whatever it falls onto.
Save up on Silt Blocks for when an Extractinator is acquired to obtain large amounts of Coins, Ores, and Gems. Along with the other Extractinator blocks, this is one of the main sources for the "alternate" pre-Hardmode ores.
Like other blocks that possess gravity, Silt can be easily harvested by digging below the block that is holding the Silt up then placing a Torch or Minecart Track under it. Then the last step is to break the supporting block & let the Silt fall in.