You are getting error because your -storing BLOB NOT NULL- column has no default value, so MYSQL refuse to do.

INSERT INTO `cover` (id) VALUES (…);
there’s no value for storing and no default to fall back on.

You have a simple ways to fix it:
1. Allow NULL (or give it a DEFAULT NULL)

If you don’t mind that a newly-inserted row comes in with no image yet, alter the column to accept NULL:

"ALTER TABLE cover MODIFY storing BLOG NULL DEFAULT NULL";

After that, your INSERT INTO cover (id) VALUES (…) will work, and you can UPDATE … SET storing = … later.