Delete sql rows where IDs do not have a match from another table

DELETE FROM blob 
WHERE fileid NOT IN 
       (SELECT id 
        FROM files 
        WHERE id is NOT NULL/*This line is unlikely to be needed 
                               but using NOT IN...*/
      )

Using LEFT JOIN/IS NULL:

DELETE b FROM BLOB b 
  LEFT JOIN FILES f ON f.id = b.fileid 
      WHERE f.id IS NULL

Using NOT EXISTS:

DELETE FROM BLOB 
 WHERE NOT EXISTS(SELECT NULL
                    FROM FILES f
                   WHERE f.id = fileid)

Using NOT IN:

DELETE FROM BLOB
 WHERE fileid NOT IN (SELECT f.id 
                        FROM FILES f)

Warning

Whenever possible, perform DELETEs within a transaction (assuming supported - IE: Not on MyISAM) so you can use rollback to revert changes in case of problems.

DELETE FROM blob
WHERE NOT EXISTS (
    SELECT *
    FROM files
    WHERE id=blob.id
)