Max and China: A Punnett Square

Meet Max.
We got him when he was just a couple of weeks old, almost 10 years ago.

Meet China.
She belongs to the neighbors and has been Max's girlfriend for the past 6 years. We do not let Max out without a chain (because he runs away for fun), but the neighbors do let China out. They see each other usually on the afternoon when we chain Max in the front yard so he has a change of view (how spoiled!).

A a couple of years ago, China managed to sneak into our backyard and ended up carrying Max's offspring. Four puppies were born, and they looked something like this:

Yay for Photoshop cloned puppies!

Let's assume that the dominant allele for black coat is "B", and the recessive allele for the light beige coat is "b". Looking at Max's phenotype, we can deduce that his genotype is homozygous recessive, bb. But in China's case, her phenotype doesn't make her genotype as obvious.
China can be either homozygous dominant, BB, or heterozygous, Bb. Let's find out which is the correct genotype:
If China's genotype was homozygous dominant, BB, there is no way a puppy with light beige coat could be born. All the puppies would be heterozygous: black, carrying the recessive allele for beige coat.
Now we can see that the only way a puppy would have beige coat would be if China was heterozygous.