first of all, I don't think that BLOB and VARCHAR2 are compatible, logically that is. BLOB is binary and VARCHAR2 is mere text.
The way I'd do it is to have all the VARCHAR2 saved into text files and then stored in BLOB. Here is a hint:
let's call your VARCHAR2 column 'X'
1- create a new column in the table of type BLOB and call it 'Y'.
2- using pl/sql text I/O, save each VARCHAR2 in a text file using the primary key of the table as file name.
3- upload each file back to the table in it's corresponding record in column 'Y'.
4- update the table setting 'X' in each table to NULL.
5- alter table to delete column 'X'.
6- alter table to rename 'Y' ---> 'X'
finally, before doing any of these steps make sure you have enough disk space and that you really know how to deal with BLOB's as they need special insert and update commands.
i read what you say and i found it interessant for me because i have the same problem
Si i hope if you can, to help so mine consisting of
first i have an image (blob) that i can affich here but i would like will be converted to text for using this text as temporary file which can be shown when i want