If you go into Aldi's, or any major grocery store really, they will have a selection of dark chocolate that has different levels of cacao content. Chocolate is basically cacao and sugar. Milk chocolate has lots of sugar and just enough cacao to give it some taste. It is a chocolate flavored sugar bar.

How bitter it is basically is how little sugar it has. Bakers chocolate that is almost 100% chocolate is very bitter. 85% cacao (which my wife usually uses for making our low carb chocolate deserts) is about 15% sugar (something actually less than that but without getting a bar out and looking I am only guessing). This is pretty bitter for most people. I forget where the break is but somewhere around 50-60% cacao and up is considered "dark" chocolate. So you might look for something advertised in that range. It still has lots of sugar but not nearly as much as milk chocolate.

For health, the less sugar the better. For taste, the more sugar the better. Pick your poison. 

PS: The Hershey little dark chocolate bars that you get at Halloween, they have "lots" of sugar and are really only on the edge of being dark chocolate.