I bought a blue heeler puppie. Recently she started loosing the blue color, and has turned dark black.?

I recently bought a Blue heeler puppie. She is pure breed, and had all her colours as a young pup. However, recently she lost most of her color and is now dark black and tan. Has this happened to any of you guys? I hardly doubt that she is not a pure breed as she has all the ACD instincts, bodily features and intelligence. She just simply turned black. Any ideas?