>>95885142
To build a black Black Templars army youstart with the Space Marine Codex. Then you apply the Black Templar rules on top of that. You can't combine any of the Space Marine chapter rules, they are mutually exclusive.

You can then pick
>vanilla space marines (you get inmproved oath of moment)
>deathwatch
>blood angels
>space wolves
>black templars
>raven guard
>iron fist
>iron hands
>ultramarines
>dark angels
>white scars

If you do, you can't include any units in your army from any of the others. If you are army is one of the ones with special rules like Space Wolves, Black Templars etc you can then select one of the detachments included in their Index/Codex.

The rumored Space Marine Codex 2 supplement thats supposed to release this fall will include new detachments that let you play as the first founding chapters that don't have any special rules of their own.

So a Black Templars player would select Adeptus Astartes as their Faction Keyword in army creation, then choose a Black Templars detachment (currently there are two: Righteous Crusaders or Wrathful Procession). They would have access, like any Space Marine army, to take Black Templars units, but doing so would lock them out of any other special space marine units. The BT detachments will provide the rules and buffs that make the BT units actually useful.