The term "Dark Chocolate" is so misused it drives me mad! It's just a general term for both
bittersweet and semi-sweet. The 54% cacao solids would usually be in the semi-sweet category, although, there have never been any strict standards established. One brand's bittersweet chocolate is often another brand's semisweet. I use 60% in general when a recipe calls for bittersweet, unless otherwise indicated, such as using a 70% chocolate, etc. Also, 60% is the general standard for baking with bittersweet, because the higher percentages will yield a drier result if they're not specifically indicated.
If you use the 54%, it will be sweeter than using the listed bittersweet. I've tried the Pound Plus, and I find it pretty sweet. But, you can definitely use it as a sub for bittersweet.