White hairs and black spots

[RhndLev@cs.com]

Someone on the list mentioned that large areas of white hairs can be due to 
heat build-up.  I purchased a Tennessean saddle for my gaited horse and it 
appeared to fit well.  She showed no signs of soreness and gaited very well 
in the saddle.  However, when her winter coat came in, there were large, 
symmetrical areas of white hairs just behind her withers.  I looked on the 
underside of my saddle and in the area of the saddle over those areas of 
white hairs, the saddle fleece is completely black.  Now, it wasn't like that 
before and my saddle pad is hunter green, so I don't think that it comes from 
the saddle pad.

Does this seem weird to anyone else?  Is it true that large areas of white 
hairs are not as big of a deal as small areas?  I've stopped using this 
saddle until I know more.

Rhonda