The block drops overview can be used to configure the items that will be dropped when a custom block is destroyed. If your block simply has 100% chance to drop a black_stone block/item, it should look like this:

On the left, there is

• a Back button. Clicking it will take you back to the block edit menu of the block you were editing.
• a Search field. If you type text in there, only the drops whose description contains the text you type will be shown. This could be useful if your block has many drops.
• an Add drop button. Clicking it will take you to the block drop edit menu, where you can configure it.

The rest of the page is used to display the current list of block drops. Note that the block drops displayed here are somewhat more complicated than you might expect: each block drop can drop multiple items and have some conditiosn like required items. For each block drop, there is

• a description/summary on the right of an optional texture.
• an Edit button on the right of the description. Clicking it will take you to the block drop edit menu, where you can edit it.
• a Copy button between the Edit and Delete button. Clicking it will take you to the block drop edit menu, where you can create a new block drop. But, you won't be creating it from scratch: all fields will be filled with the values of the block drop you're copying.
• a Delete button on the right. Clicking it will remove the block drop.