Hmmm... she looks like she'd be classified as black w/ white markings. I don't see any brown. There is one color type called "seal" where it looks black in most lighting but reflects brown in the sunlight; but I don't know if that name/color is found in chihuahuas. And I think I've only seen "seal" in short coat dogs.
Example: here's 2 Boston Terriers. Both look fairly similar in normal indoor lighting. But outside, you can clearly tell the one on the right reflects brown (even though the fur itself is not considered chocolate.)