Those look like maraschino cherries for embellishment which I don't believe are used in authentic black forest cakes. My great grandmother came from the Black Forest but I know very little about her or their traditions. She died of heart trouble, possibly caused by diabetes, at age 42. Many did, and they seemed to go in July in the hottest part of the summer with no means to cool, must have been miserable.
I'm making giant pearl cherry tapioca in my crock pot using just one pkg of frozen sour cherries and some slightly diluted tart unsweetened cherry juice which is very popular now.
This was my last box of large pearl tapioca, and I need to find some more, preferably not Amazon, Asian market downtown may have it if I can ever get back there.
I agree, sour cherries.
They do use candied cherries in Switzerland (and on this cske) but I think they arent those nasty maraschinos. They are harder and sugary, crinkly like dried fruit but glazed a bit.