HP Dark is not perfect IVs, but a result of perfect IVs. Hidden Power is determined by the alternating odd/even numbers of a Pokémon's IVs. For example, the IV pattern
results in Hidden Power Ice. However, this does not mean that just because a Pokémon has HP Ice it has an IV set of 31/30/30/31/31/31. Hidden Power Ice is caused by anything that follows the above pattern, like 31/12/4/31/19/23/23, or 1/0/0/3/15/31.
TL;DR Hidden Powers are a poor way of checking IVs, as they yield a pattern, not a specific number.