Aqui, dê uma chance:
select * from users where user_id <> 4 and language = 'en'
and user_id not in(select blocked_user_id from users_blocked where user_id = 4)
and user_id not in(select user_id from users_blocked where blocked_user_id = 4)
Demonstração aqui:http://sqlfiddle.com/#!2/b22a4/2
