I had been taught that freed slaves "took the last names of their masters." Wikipedia says:
After emancipation, many freedmen and women took the surnames of their former owners as their own.^[citation ^needed]
I always thought this was weird -- why would you want the name of the guy who owned and likely mistreated you?
But perhaps the freed slaves didn't have a choice, and it wasn't so much "taking" a last name, as one was given to you?
Typically, your last name as a slave would be your master's last name because it identified you as his property. If you've been called something your entire life, even with the bad implications of it, most people wouldn't bother changing their name.