This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori 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.