Blocks, block devices and block sizes must be considered together.
What is a block device
A block device is a device you can read blocks from. For example hard disks, cdrom drives and floppies are block devices, but not the keyboard. You can receive data from the keyboard and regard them as blocks, but you cannot seek on the keyboard. You can tell a hard disk "give me block 5433", then block 7707, then block 1807 and you cannot do this with a keyboard, so, a keyboard is no block device.
It is important to understand the ideas behind the block sizes.
Disk block sizes
Typically, a hard disk cannot read less than 512 bytes, if you want to read less, read 512 bytes and discard the rest. This is why dd reads one block à 512 bytes in the following example:
tweedleburg:~ # dd if=/dev/sda1 of=/dev/null count=1 1+0 records in 1+0 records out 512 bytes (512 B) copied, 1.8977e-05 s, 27.0 MB/s
File system block sizes
tweedleburg:/mnt/sdb2 # stat -f . File: "." ID: 236d62321492c2ce Namelen: 255 Type: ext2/ext3 Block size: 4096 Fundamental block size: 4096 Blocks: Total: 76899893 Free: 8380419 Available: 4474114 Inodes: Total: 39075840 Free: 38013010
So, if you store a file in this file system, it will be stored in a 4096-byte-block, that means, even if your file only contains 5 bytes, it will take away 4096 bytes from your disk's capacity:
tweedleburg:/mnt/sdb2 # df . Filesystem 1K-blocks Used Available Use% Mounted on /dev/sdb2 307599572 274077896 17896456 94% /mnt/sdb2 tweedleburg:/mnt/sdb2 # echo hallo>welt tweedleburg:/mnt/sdb2 # df . Filesystem 1K-blocks Used Available Use% Mounted on /dev/sdb2 307599572 274077900 17896452 94% /mnt/sdb2 tweedleburg:/mnt/sdb2 # du -csh welt 4.0K welt 4.0K total
Kernel block size
Also the kernel has its own block size. This is relevant e.g. for vmstat. In the vmstat man page you find the statement
All linux blocks are currently 1024 bytes.
So, again another block size when you work with vmstat. This is the block size the Linux kernel uses internally for caching and buffering. It is the most prominent of all block sizes.