SQL & PL/SQL

I was going to add some error handling into a procedure, where someone had either not filled a BLOB column in a table, or if they had filled it with empty_blob(). I can't figure out how to test for empty_blob() though ... as I keep getting errors. The tests which failed:

if l_blob = empty_blob() then ... 

and

declare 
  l_test_blob blob := empty_blob();
begin
 ...
  if l_blob = l_test_blob then ... 

Also, I've not been able to find much on what empty_blob() actually accomplishes. If someone has a better link than the really short descriptions I've found so far, it would be really appreciated.

Thanks,
--=Chuck