1) Black metal is first and foremost defined by Black Metal Ideology. It MUST have to do with Satanism. All others can be labeled "black-influenced" but cannot claim the title of black metal.
2) Black metal is secondly defined by a specific sound. This sound is best defined by DMDS and early Darkthone (and the first two Burzum albums which were actually black metal) releases. The musical part is more flexible than the ideology. Because of this, bands like Deathspell Omega can still claim to be black metal because of their intensely orthodox Satanism.
3) Something cannot claim the tag of "black metal" unless it conforms to both of the above.
These are axioms of black metal. They are not to be questioned.
This explains why bands such as Ondskapt and Watain deserve this title.
This also explains why Burzum's Hvis Lyset Tar Oss & Filosofem are not black metal (inconsistency in ideology).
This also explains why Borknagar, Enslaved, Forest Stream, Agalloch, Wintersun are NOT black metal.
since there will always be people who disagree, I will not argue this further, however it is imperative that I point this out, that there is an unquestionable orthodoxy in the concept of true black metal that must not go unignored.