I have read posts that say white chalk works pretty well. It's kids' chalk, and you can sharpen the point with a pencil sharpener. Haven't tried it myself, though.
Beware of the blue markers -- make sure you rinse out *thoroughly* with water (not just misting). I unpacked a white muslin top I had marked with the blue pen before quilting and there are very weird brown dots in the quilting lines. I obviously thought I had rinsed it well, but clearly I had not rinsed it evenly. Parts of the top have no markings; other parts have brown dots. There is no rhyme or reason I can see to the dots, other than being associated with the quilting lines, so I assume I was not as thorough as I thought when I rinsed.
Edit: I have used white soap slivers to mark dark fabrics. The soap has to be old and dried out. Not sure how well they would work with a stencil, though. My biggest problem is that the edge of the soap would get flattened pretty fast, so my lines often ended up thick.