SQL & PL/SQL :: Extracting Blob Data Using Oracle Function?

Below is the function code used to extract data from blob column. The function works fine when the blob data length < 2000 bytes. When it is more than, it is throwing an error as below.

Table name: mr_test
Columns: id number
seo blob

CREATE OR REPLACE FUNCTION fn_mr_blob(in_id IN number) return varchar2
IS
len NUMBER;

[Code]....

ORA-01489: result of string concatenation is too long

when I replcae the

"SELECT myvar||trim(dbms_lob.substr(seo,bytelen,vstart)) into myvar FROM mr_test WHERE id = in_id;"
with
SELECT trim(dbms_lob.substr(seo,bytelen,vstart)) into myvar FROM mr_test WHERE id = in_id;
myvar1 := myvar1||myvar;
ORA-06502: PL/SQL: numeric or value error: character string buffer too small