MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE posts (id INT, title VARCHAR(50));
INSERT INTO posts (id, title) VALUES (1, 'Post A'), (2, 'Post B'), (3, 'Post C');

CREATE TABLE tags (id INT, name VARCHAR(50));
INSERT INTO tags (id, name) VALUES (1, 'Tag 1'), (2, 'Tag 2'), (3, 'Tag 3');

CREATE TABLE tags_posts  (post INT, tag INT);
INSERT tags_posts  (post, tag) VALUES (1, 1), (1, 2), (2, 3);

CREATE TABLE blocked_tags (user VARCHAR(50), tag INT);
INSERT blocked_tags (user, tag) VALUES ('user1', 1), ('user1', 2), ('user2', 3);

Records: 3  Duplicates: 0  Warnings: 0



Records: 3  Duplicates: 0  Warnings: 0



Records: 3  Duplicates: 0  Warnings: 0



Records: 3  Duplicates: 0  Warnings: 0


SELECT  *
FROM    posts AS p
WHERE   NOT EXISTS
        (   SELECT  1
            FROM    tags_posts AS tp
                    INNER JOIN blocked_tags AS bt
                        ON bt.tag = tp.tag
            WHERE   bt.user = 'user1'
            AND     tp.post = p.id
        );


id
title


2
Post B

3
Post C


SELECT  *
FROM    posts AS p
WHERE   NOT EXISTS
        (   SELECT  1
            FROM    tags_posts AS tp
                    INNER JOIN blocked_tags AS bt
                        ON bt.tag = tp.tag
            WHERE   bt.user = 'user2'
            AND     tp.post = p.id
        );


id
title


1
Post A

3
Post C


/*User 3 has no blocked tags so should return all posts*/
SELECT  *
FROM    posts AS p
WHERE   NOT EXISTS
        (   SELECT  1
            FROM    tags_posts AS tp
                    INNER JOIN blocked_tags AS bt
                        ON bt.tag = tp.tag
            WHERE   bt.user = 'user3'
            AND     tp.post = p.id
        );


id
title


1
Post A

2
Post B

3
Post C