How can I read a BLOB for which i have only read access?

Server: oracle version 11.2.0.4 on AIX

Problem:

I want to read a BLOB in a procedure and I have only read access to the BLOB.

When I only use

DBMS_LOB.READ(DatenNK,amountLob,posBlob,abloSatz.BLOB);

then I have no problem.

But when I try to open the BLOB with

DBMS_LOB.OPEN(DatenNK,DBMS_LOB.LOB_READONLY);

then I get an error because I can't change DatenNK.

And why I want to use OPEN (and Close): the literatur says that I will have better performance if I open a Blob befor i read it.

Do you have such an problem? How do yo solve it? Why I get an error during DBMS_LOB.OPEN : I only want read the BLOB?