I use the Ultra grooming chalk and it works nicely as well for small blemishes.
I use the white for my geldings markings, and then in black to cover a couple of old scars for halter.
I'd give you a link, but my computer freezes every time I go to do so, but you can find the chalk at Schneiders website.
"You're just as sane as I am."~Luna Lovegood.