I think you'll find they will do okay with the black earth substrate IMO. it is fairly light too so they are likely to push it around as needed to get the food they want in the substrate.
The concern for corys is that majority of substrate are sharp (gravels, etc) which will over time abrasively cut/destroy their barbels which do not grow back but are important to the fish.
My corys are all in bare bottom tanks and have no issues so the sifting is not a requirement IMO, it is nice ot have as its their natural behaviour though.