Your dog is absolutely gorgeous. I think she might be purebred like you think. Maybe there is border collie but maybe not. A while back I had a conversation with a lab breeder about colors, and believe it or not is is actually very normal to see black labs and brown labs with white chests and feet... and yet they are purebred, not mixes. It doesn't happen all the frequently, but it is still completely normal. Either way, she is gorgeous and looks super happy.