Blood-red Bone-Hard Blisters Children's Short Story

Hi. I need help finding what I think is a kid's short story that my sister remembers reading on one of those reading comprehension cards in elementary school during the 70's. You know, the ones that had reading levels by color, red, green, blue, goldenrod... She says it was a very scary little children's story that really made an impression on her, but hasn't seen anywhere since. Would appreciate any leads, author's name etc. The punchline of the story was something about blood-red, bone-hard blisters. Thanks ever so!