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>