Spotted on Reddit, company documentation describes the stout flavor as a "roasted malt flavor." While the drink's flavor is described as "chocolaty stout," it contains neither beer nor mocha sauce. It's available hot, iced, or as a Frappuccino.
The Dark Barrel Latte appears to be testing in Columbus, Ohio and in, at least, a few locations in Jacksonville, Florida.
Dark Caramel Latte pictured via Starbucks.