I really like this beer. Made it a few times now. You hould ry it.
Hey, that's my recipe! I think it's great. I brew BIPA a lot, in part to have two IPAs on tap that are visibly different.
A few things to consider: at this quantity, you will get some flavor from any roasted malt product - carafa spezial II, midnight wheat, sinamar, and blackprinz all have different flavors they provide depending on how you use them. Sinamar will give you the smallest flavor/color ratio. Adding the dark malt to the mash will provide the largest flavor/color ratio.
I like to get the right amount of dark malt (Carafa Spezial II or III) to put me at ~40 SRM, but I add it all at sparge. I batch sparge with coolish water (~150º F)when I make black IPA and let the sparge water steep for 10+ minutes to extract enough color. When the recipe says 40 SRM, I usually get about 30-35 SRM from this process (which is ample). Generally, 1.5 lbs per 6 gal works.
Since you are adding that dark malt flavor, you need to consider other things in recipe development. Going heavy on the sulfate will make the beer more mineral-bitter, which will make people (even experienced BJCP-certified judges) think the beer is too roasty. Don't do this. Keep the mineral profile minimal and balanced. An agressive bittering hop will do the same thing. Use something really smooth like Summit, Apollo, Magnum, or a resin product like HopShot. I like the addition of a little light or medium crystal malt and munich malt in this style - roast by itself will be more prominent than roast balanced by a little "sweet" flavors (though I make it very dry by mashing low and using an American yeast). After that, just brew it like any other IPA. Personally, I like the hop combo of Simcoe + Citrus (Cascade, Amarillo, or Centennial), but whatever tastes great in an IPA will taste good in a BIPA - though it may come across as less fruity due to the smooth roast malt flavor you are getting.