Hmm...some people withhold things like roasted barley or black patent til very late in the mash or sparge if the whole purpose is just to get color...
In my opinion, there are easier ways to get color dialed in like sinamar or Brewers caramel...
But in this case I would just add it all at the beginning of the mash as roastiness and dark blackness is the whole purpose of this beer.
It will not be overwhelming...
Just mind your water has some hardness to it, or the beer will come out tasting a bit sour in my past experience...
Good choice brewing this style! I've never had one come out bad despite my past sins in royally botching the brew day