Hummm. "mixed" is not the word that comes to my mind when looking at that picture. Nor would I call Kelly Rowland "dark-skinned". Descendents of slaves are all mixed and they come in many shades ranging from high yellow to deep brown. Facial features and hair texture are also traits that figure into the equation. None of this, however, matters to white folks who have an affinity for the word "nigger".
Typically, "Afro-Americans" tend to use their own skin color as the criteria when categorizing others of their race. This is a very subjective measuring stick and varies from person to person. So it's all about opinion.
Dating back to the time in the mid 1960s, thanks to Malcolm X, the term "negro" was replaced by the title "black" which previously carried a stigma. This was when the concept of blackness being "a state of mind" was introduced.
I'm not sure what the current mind set is...