I found those fabrics about 15 years ago in Hungary. Since this time I go there twice a year. They are done in quite a simple way: First of all the white linen gets printed (by hand) with the so called "papp" which is kind of wax. This prevents the Indigo from getting on certain areas on the fabric. After the dying process which takes place several times the "papp" is removed and the dyed fabric is done. Those indigo-dyed fabrics are usually totall washable.
I have a online - shop for those fabric here in germany. You can contact me. Can I post a link here?