$mysqli query fails, foreign key constraint fails - php

Question

This insert command fails:
INSERT INTO posts (id, headline, content, post_date, blog) VALUES (default, 'Birds
in Nepal are being Messed With', 'Have you heard about this? There are birds in
the country of Nepal that are sitting around minding their own business, when out
of nowhere they are getting messed with. Messed with!', curdate(), 'cost');
It dies with this error message:
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint
fails (`allanwebdev`.`posts`, CONSTRAINT `posts_ibfk_1` FOREIGN KEY (`blog`)
REFERENCES `blogs` (`blog`))
There are two tables, one called 'blogs' and one called 'posts'.
'blogs' has one column called 'blog' which serves as a primary key:
DESCRIBE BLOGS;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| blog | varchar(10) | NO | PRI | | |
+-------+-------------+------+-----+---------+-------+
SELECT * FROM BLOGS;
+----------+
| blog |
+----------+
| code |
| politics |
| satire |
+----------+
'posts' has a foreign key column called 'blog' which references the primary key in 'blogs'.
DESCRIBE POSTS;
+-----------+----------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+----------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| headline | varchar(60) | YES | | NULL | |
| post_date | date | YES | | NULL | |
| content | varchar(64000) | YES | | NULL | |
| blog | varchar(10) | YES | MUL | NULL | |
+-----------+----------------+------+-----+---------+----------------+
What. The heck. Am I doing wrong? Many thanks. This error message does exist elsewhere but invariably they are more complex problems. I'm operating simply and directly from a mysql command line.
Note that the sql call is being made from a php mqsqli object $db->query($sql). The actual string doesn't include a semi-colon at the end.