Actually this is quite possible, as the coat color in Labrador Retrievers is controlled by two separate loci- B and E. The "E" locus controls dilution and causes yellow labs to have their coat color.

"B" represents the dominant trait, and whenever it is present (with the exception of dilution in yellow labs) the dog is a black lab. "b" represents the recessive trait, and whenever the genotype is "b b" (with the exception of dilution in yellow labs), the dog is a chocolate lab.

Black Labs' genes would look like this: B _ E _
Chocolate Labs' genes would look like this: b b E _
Yellow Labs' genes would look like this: _ _ e e

So, in the case of the picture the mother would have had the genes _ _ e e.
The father could have been either a black lab or a chocolate lab, but if he was a chocolate lab the mother's genes must have been B _ e e.

I actually took a class last semester that focused completely on genetics, and this was the only part we studied for a third of the class.

(There is also a second type of yellow lab, which is unregister-able and called "Dudleys". Their genotypes are bbee, and they have either pale or chocolate pigment or an absence of skin pigment. But the above female is obviously not a Dudley.)

tl;dr Yellow bitches can have black pups, even when bred to a chocolate dog, and this is because of genetics.