What Other Breed Is This Dog?

I have a dog named Lily. She is going to be 2 years old sometime at the beginning of June. We were told she was full Chihuahua, but based on her coat color pattern, we aren't sure if she fully is. When she was a puppy, she was fully black with a white tummy and some edges of her paws were white too. It was sort of like a tuxedo pattern.

As she grew up, white/blondish hairs began sprouting around her neck and along her back. Also if you look at her fur closely, it looks like brown hair can be found too. I just am very curious about what she could be mixed with. Also she is bigger than a Chihuahua's normal size and she weighs about 15 pounds.

By annamay3022