“Negress”? What is this, the 1920s?
To determine whether something is “cultural appropriation”, ask two questions:
1. Are you misusing symbolism by putting it outside of context, or using it in a way that actual members of the culture would find shallow, insulting, and/or dehumanizing?
2. Are you profiting from symbolism in a way that members of the culture are oppressed for doing so, or worse, profiting at their expense?
If either is “yes” then it is. If both are “no” then it almost certainly isn’t. There is no #3, “you are not a member of the race most common to the culture in question”. Sometimes cultural outsiders have been forbidden from doing (or occasionally, even knowing) certain things and this could be considered a form of misuse, but I would be very surprised if this ever applied to Irish step dancing.
I admit to not knowing much about Morgan Bullock, but from the little I’ve seen, it doesn’t appear that what she’s doing is appropriation.