Вы прочитали что-то, что кажется неправильным.
Согласно документации Oracle 10g Release 2 :
The BLOB datatype stores unstructured binary large objects. BLOB objects
can be thought of as bitstreams with no character set semantics. BLOB
objects can store binary data up to (4 gigabytes -1) * (the value of the
CHUNK parameter of LOB storage).
If the tablespaces in your database are of standard block size, and if you
have used the default value of the CHUNK parameter of LOB storage when
creating a LOB column, then this is equivalent to (4 gigabytes - 1) *
(database block size).