Hi Yang,
you are aware that this is not any SAP internal forum, but open to everybody, yes? 
Anyhow, check the documentation for the BINTOHEX function:
Syntax
BINTOHEX (expression)
Description
Converts a binary value to a hexadecimal value.
A BLOB is not a binary value. It's a data type that can store binary values.
Try something like
insert into a_column
values (1, x'F9')
SELECT BINTOHEX(to_binary("F9")) FROM A_COLUMN;
Works for me...
- Lars