Ladies, I'm still using chalkboard chalk - easy, washes out, inexpensive. Am finding it is perfect for all of the fabrics I have used.
Even used it on an expensive dress I got off of the sale rack (is there any other way to buy????) to mark my hem and it came right out.
Found that the light colors (pink, yellow, blue, green) work good on light fabrics. White on the dark ones.