The normal approach for Black Legion Cult Troops (Khorne Berzerkers, Thousand Sons, etc...) would not be to paint them completely black but to paint them in their 'standard' paint scheme and just give them the symbol of the Legion the Eye of Horus, sometimes not even this. This is because of the various pacts the leaders of the Black Legion have signed with various other way smaller warbands to strengthen their might.
So typical Black Legion warband of the 13th Crusade could look like the following: A big group of Black Legion
CSMs with Daemon engines, some Khorne Berzerkers from the Skulltakers, multiple units of Night Lords Raptors and a small group of Iron Warriors being Havocs or driving tanks.
For the main paint scheme it's most important that YOU like it. You will spend hours to paint your tiny horned movie villains, the time your buddies have to deal with them, is puny compared to this, so make sure you are happy with it.
If you don't want to make Word Bearers (We're the Mormons of
40k after all, so I have to ask

) you should probably make your own renegade chapter this would leave you the most freedom both in fluff and painting.