Custom permalink for one category

Your rewrite rule doesn’t capture anything. Change:

'^blog/?'

to add the capture group you reference with $matches[1] in your rule:

'^blog/([^/]+)/?'

visit the Settings > Permalinks page, which will flush rules, and your code should otherwise work.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)