Cobalt Block is one of the many Decoration Blocks added by the Soul Forest Dimension. It can also be called a Storage Block. The Cobalt Block emits light after its placed, it doesn't need a redstone signal for that.
Aswell as most of the Decoration Blocks this block is crafted with 9x Cobalt Ingots. After a Cobalt Block is made, its content (the gems) can be easily re-obtained by simply placing the block in a crafting table.
As the block emits light it can be used in a lot of ways, a good thing about it is that it doesn't need a redstone signal to emit light.
A different use, but most likely a much more usefull function is the fact that you can easily store a lot of Cobalt Ingots by turning them into blocks.