There are several copys of your post on here so wasn't sure which to respond too:
The white patch is from a pressure point on your saddle. If you just got the horse it could be from the previous owner as it takes awhile for the white hairs to show up. You may need to check your saddle fit and placement.
I want to add that my black horse will get little bits of white hairs between seasons (like now) when she is shedding out or growing a winter coat. Not like the white patch from the pressure point but just a hair here and there.
Also check out http://www.mastersaddlers.com/points.htm
it gives good suggestions for checking your saddle fit.