SELECT 
  CONCAT(u.firstname, ' ', u.lastname) as author, 
  b.page_id 
FROM 
  cscart_blog_authors as b 
  LEFT JOIN cscart_users as u ON b.user_id = u.user_id 
WHERE 
  b.page_id IN (51, 49)

Query time 0.00037

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00707478,
    "nested_loop": [
      {
        "table": {
          "table_name": "b",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["page_id"],
          "loops": 1,
          "rows": 2,
          "cost": 0.0036475,
          "filtered": 100,
          "attached_condition": "b.page_id in (51,49)"
        }
      },
      {
        "table": {
          "table_name": "u",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["user_id"],
          "ref": ["u985510652_ecartify_dev.b.user_id"],
          "loops": 2,
          "rows": 1,
          "cost": 0.00342728,
          "filtered": 100
        }
      }
    ]
  }
}