Semisweet chocolate is generally about 60% cacao, while bittersweet is more like 70%. There are subtle differences, which is why, although many people can’t tell the difference, your more refined palate picked up on the bitter notes.
Oh, and while this site is awesome, urgent kosher foodie questions often go to one of the kosher foodie blogs, where there are lots of folks who would notice it right away & jump in to help.
You didn’t mention how you planned to use the chocolate so hatzlacha rabbah with whatever you do.
Chag someach 🙂