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 = 42

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "b",
      "access_type": "ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["page_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100,
      "using_index": true
    },
    "table": {
      "table_name": "u",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["user_id"],
      "ref": ["fluentmart_cscart.b.user_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}