How big is a 3DS storage 'block'?

Blocks on the 3DS are each 128KB in size. This translates to 8 blocks per Megabyte, or 8192 per Gigabyte.

_{(All using proper binary prefixes, of course.)}

According to various google results 1 block is 128kb.

Using blocks to describe volume capacity isn't very uncommon. Blocks on hard drives are usually known as sectors. This isn't transparent and an end-user does not need to know about this, especially as the physical layer on the harddrive is further abstracted by the overlaying filesystem with its own restrictions on allocation sizes.
I assume that Nintendo uses blocks to describe minimal allocation sizes, so a file that is smaller than 1 full block will still use one full block of space as there is no means of allocating let alone accessing partial blocks.

[A] block [...] is a sequence of bytes or bits, having a fixed length (a block size). [...] Blocked data is normally read a whole block at a time.

The size of one storage block is 128 kilobytes. This means you get 8,192 blocks per gigabyte.