I have brewed a lot of black IPA and I have strong opinions on the subject. First of all, in the amounts needed, even the least roasty dark malts will give plenty of flavor. That means you really really want to use the gentlest-tasting roast malts (Carafa Special II or III, Midnight Wheat or Blackprinz). Shooting for 50 SRM is pointless. There is no difference between 40 SRM and 50 SRM. Frankly, I shoot for 35-40 and it is pitch black. Here's my usual malt bill (it varies, but this is basically it):
80% 2-row
6% Munich
6% C-40
8% Carafa Special III (added at sparge)
The technique I use it to sparge a little cooler than usual (maybe 150F) and add the dark malt then. I batch sparge, so really I add the dark malt right as I begin to runoff the first runnings, then I add the sparge water, give it a stir and about 2 minutes, then run it off into the kettle.
Another consideration: all that roast malt will increase perceived bitterness. I bitter my IPA about 15-20 IBUs less than I would want if it weren't black. In this beer, I would bitter it to no more than 60 IBUs, assuming standard AIPA strength, and expect it to taste like 80 IBUs because of the dark malt.