We have german-umlaut characters stored in a BLOB field.

We have german-umlaut characters stored as a BLOB.  For example we store this text in the field:  Hohe TAN. Das Öl ist bedingt gebrauchsfähig. Ölwechsel bei Probenahme: Ja

When we use the statement below to extract the text:

select UTL_RAW.CAST_TO_VARCHAR2(dbms_lob.substr(BLOB_FIELD,100,2))

  from BLOB_VALUES

  where BLOB_ID = 319905;

It returns instead of the way it was entered/stored:

"ohe TAN. Das � ist bedingt gebrauchsf�g. �wechsel bei Probenahme: Ja"

Our character set are:

NLS_NCHAR_CHARACTERSET - AL16UTF16

NLS_CHARACTERSET - AL32UTF8

What have done incorrect to retrieve the data?  Is there some other character set we need to use?