APHA calls it a "blue zone" around a marking, and they consider it a Paint trait for registration classification.
APHA.Com - Color Reguirements
IMO all it really is, is an overlap of the white hair over the dark skin at the border of a marking.
As other post-ers have mentioned, Appaloosas sometimes have a similar type of bordering-- called a halo, around their dark spots-- interestingly, most foals and yearlings with Appaloosa spots don't have halos surrounding them--- it seems to develop later. In other words, just because they don't have halos as a young horse does not mean they won't have them as a mature horse. I wonder if the samething is observed with the blue zone/mapping etc. aound paint/pinto spots?