BLOB[(M)]

A BLOB column with a maximum length of 65,535 (216 − 1) bytes. Each
BLOB value is stored using a 2-byte length prefix that indicates the
number of bytes in the value.

An optional length M can be given for this type. If this is done, MySQL
creates the column as the smallest BLOB type large enough to hold
values M bytes long.

URL: http://dev.mysql.com/doc/refman/5.7/en/string-type-overview.html

