I really hope this is a temporary tattoo. But I don't think it is. Gertie in Queens, New York has a post on her blog http://www.blogforbettersewing.com/ where you can read (and see) about sewing tattoos. It's a very interesting blog about vintage fashion.
P.S. My sister Lynne says this is a temporary tattoo. She enlarged the photo and looked at the edges; also the tat is shiny on her left collar bone.
P.P.S. FlossieBlossoms says she thinks this is a real tattoo. She saw progress shots on the Dainty Squid blog. Thanks, Floss.