EMPTY_BLOB() stores an empty BLOB:

CREATE TABLE my_blob(
        id INTEGER PRIMARY KEY,
        blob_column BLOB NOT NULL
);

INSERT INTO my_blob (id, blob_column) VALUES (1, TO_BLOB('1001'));
INSERT INTO my_blob (id, blob_column) VALUES (2, TO_BLOB('A0FF'));



SQL> UPDATE my_blob
  2  SET blob_column = EMPTY_BLOB()
  3  WHERE id = 1;

1 row updated.

SQL>