One doesn’t really come across many Belgian-styled black IPAs, let alone imperial ones. It’s not often (given what I do for a living) that I find myself drinking a style, weirdo ingredients aside, where I’m like… I really don’t think I’ve had one of these things before, possibly ever. The individual parts are super familiar: Belgium + Black IPA. The combination, a bit less so. These beers often combine firm hop bitterness with assertive Belgian yeast strains (offering notes like pepper, clove, vanilla and fruit). Occasionally there’s dark chocolate, or caramel.
Aside from the Troubadour Westkust, there aren’t many options of this sort available in the states. Brasserie de la Senne’s Brusseleir Zwet IPA might be most common, and I would be inclined to try anything from the folks who made Zinnebir and Taras Boulba. De Struise in Oostvleteren released a barrel-aged blend along heftier lines: imperial IPA + imperial stout.
Ooh, also: De la Senne does another black IPA called Black in Japan, which I have had now that I’m looking at the thing: a Brussels-born black IPA that’s crisp, toasty, and nicely bitter.
Brussels Beer Project also does a “Belgian Black IPA” called Dark Sister, which is kind of a weird name and it may or may not be available outside of limited parts of Europe and Asia.
As far as options from elsewhere: Sierra Nevada’s Beer Camp series featured a Belgian-style Black IPA a while back, but that one seems pretty much kaput. On a smaller scale, there are at least a few dozen examples of Belgian-style black IPAs brewed in the U.S., so keep an eye out if you dug the Westkust. Also of particular note: Great Lakes Brewing (the Ontario one) has a beer that’s apparently called Only 30: The Only Black Belgian Milk IPA in Existence.
There is also, as you may have guessed, an imperial version.