I check several times per week, but still "coming soon" and "soon" is rather subjective.
Could be the $20 per sold copy just isn't enough to cover the porting expenses? If there's a financial issue with the Linux port, charge $40 for the Linux version, I'd still buy it. And I'd buy "The Collector's Edition" with both games 2-3 years from now too.
If Black Plague is half as good as Overture and the gameplay is 20+ hours I'd say it qualifies as a Triple-A title, easily contending with the other $50-$70 games on the shelves.
Big cheers to the programmers and story writers of Penumbra Overture! I haven't found any bugs/glitches and gameplay and story works well together. Good teamwork!