Silver Block is one of the many Decoration Blocks added by the Soul Forest Dimension. It can also be called a Storage Block.
Aswell as most of the Decoration Blocks this block is crafted with 9x Silver Ingots. After a Silver Block is made, its content (the ingots) can be easily re-obtained by simply placing the block in a crafting table.
The main use of the Silver Block is the fact that the portal to the Soul Forest Dimension will have to be build out of Silver Blocks. The other use is the that you can easily store a lot of Silver Ingots by turning them into blocks.