When the length of the block is specifically known, more precise terms are used: for instance, a cipher in which pairs of letters are substituted is bigraphic.
In theory, there is some overlap in these definitions; one could conceivably consider a Vigenère cipher with an eight-letter key to be an octographic substitution.
However, encryption is very difficult to perform by hand for any sufficiently large block size, although it has been implemented by machine or computer.
However, the security boost is limited; while it generally requires a larger sample of text to crack, it can still be done by hand.
In addition, even in a plaintext many thousands of characters long, one would expect that nearly half of the digrams would not occur, or only barely.