I've used Blurb's premium paper. It's a little thicker than standard and less translucent.
Blurb's dark shades are murky too, in my experience. Other than that, quality is quite good for digital printing. I've printed books on an offset printer and the quality there is much higher. But that was a run of a thousand plus books.
I haven't tried Blurb's specialty papers. They are ultra expensive.