SELECT 
  CONCAT(u.firstname, ' ', u.lastname) 
FROM 
  cscart_blog_authors as b 
  LEFT JOIN cscart_users as u ON b.user_id = u.user_id 
WHERE 
  b.page_id = 7

Query time 0.00292

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE b ref PRIMARY PRIMARY 3 const 1 Using index
1 SIMPLE u eq_ref PRIMARY PRIMARY 3 hobilifedt.b.user_id 1

Result

CONCAT(u.firstname, ' ', u.lastname)
Admin Admin