The Mysterious Disappearing Beet Color

So I bought a bunch of beets for my slow cooker borcht (beet & potato soup) recipe (again from "Fresh from the Vegetarian Slow Cooker").  They looked completely normal on the outside -- all lovely and red.  No signs indicated that they were any different from regular beets, but when I cut them open at home, they were pink and white striped inside!  A beautiful color, and one I'd never encountered before.  Here they are in the cooker, looking a little like peppermint candy:

To my great disappointment, the beets lost all their red color when cooked, and the soup ended up a dull brown.  Even worse, there was a lingering bitter taste to the soup.  Can anyone enlighten me?  What kind of beets are these, and how can I make them less bitter?  I added some lime juice to the cold soup the next day, and that helped a little, but I miss the usual sweet flavor of beets.

Slow Cooker Borcht

1 - 2 lbs. beets, peeled and chopped
1 small yellow onion, finely chopped
1 small carrot, finely chopped
1 small red bell pepper, finely chopped
1 large potato, peeled and diced
4 cups vegetable stock
juice of 1/2 lemon
2 tsp sugar
1 tsp dried thyme
1/2 tsp ground cloves
salt and pepper to taste

Combine all ingredients in a slow cooker and cook on low for 8 hours.  Refrigerate and serve well chilled.  Serve with dill.