This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I think you are missing a couple of steps, and maybe have a couple backwards (or maybe you didn't post all of your code???). I cannot see where you are defining your blob locator, nor the information that you plan on sending.
If you could post all of your code that pertains to the blob, I bet we could get you up and running in no time!
Surya, Pardon me if I am stating the obvious, but according to the error message you have posted, column BLOB, in table BLOB, owned by user SCHEMA is a mandatory (or "not null") column, and you are trying to set it to null.