I used for my thesis the \blacktriangle and \blacktriangledown commands and they work. I used the book document class.
Instead now I need to replicate them on a presentation, using the beamer document class. However in such a case the symbols do not appear and no related error seems to occur within the log.
EDIT: the symbol goes in conflict with the package unicode-math. However, when removing this package my colored bullets in math mode stop working: $\textcolor{green}{\textbullet}$.
EDIT 2: solved by using \bullet instead of \textbullet.
amssymb. I'm not sure that this package is always loaded inbeamer.