I don’t know what you are trying to do. I suppose it is some kind of test.
Also, I don’t see much relation between the question title and the code you posted.
<b>You are getting the error because your source blob is null.</b>
I would recommend taking a look at this:
<a href=”http://soft.buaa.edu.cn/oracle/bookshelf/oreilly/bipack/ch08_03.htm”>DBMS_LOB Interface</a>
But I think you are completely new to PL/SQL. If that is true, I think you should start learning the basics before trying to manipulate blobs.