The essence of black IPA (also known as Cascadian dark ale) largely overlaps with American IPA. The main and often only difference lies in the malts, where, as the name suggests, a fair quantity of dark malts is used, giving the beer its characteristic dark to very dark colour.
Despite the use of dark malts, black IPA doesn’t typically exhibit a roasted/burnt flavour.
Black IPA is a branch on the large IPA tree that began growing in the early 1990s through experimentation with different malts.
It is certainly not one of the more popular types of IPA, and these days, it can be quite hard to find.