Depending on how dark the dark is, I have had success using the Crayola washable markers to mark. (I think the fine tip ones are plenty fine enough, at least for me anyway.) I have the "bold colors" pack and the raspberry color worked fine on a dark blue I was quilting. I haven't tried soap, but but have used regular school chalk like mentioned above...it works fine but if you are using a template it's hard to keep the point sharp enough to fit in the groove w/out breaking.
luvstoquilt- wish I were in IL, I'd come to your group!