Did you get the marks out?
I marked with blue chalk a wallhanging just last night. I didn't like how the design looked so I took a toothbrush to wipe it off - like I do with the white chalk. Oh NO! It just seemed to spread and blur and make it worse! Tried wiping off with a damp cloth. This is on an off white background. A friend recommened OxyClean so I will give that a try.