How are you setting blob to DB? You should do:
//imagine u have a a prepared statement like:
PreparedStatement ps = conn.prepareStatement("INSERT INTO table VALUES (?)");
String blobString= "This is the string u want to convert to Blob";
oracle.sql.BLOB myBlob = oracle.sql.BLOB.createTemporary(conn, false,oracle.sql.BLOB.DURATION_SESSION);
byte[] buff = blobString.getBytes();
myBlob.putBytes(1,buff);
ps.setBlob(1, myBlob);
ps.executeUpdate();