Blueberries causing constipation?

My 1 year old has been on a blueberry kick lately and while her stool was always solid, it’s been soft. Since she’s started eating blueberries she’s really been struggling with harder, larger stools. I thought the opposite was supposed to happen? She eats a variety of foods and drinks a lot of water. I’ve been trying to avoid juice, but if it will help her have softer stools, I’ll try it. Anyone else have any tips?