"Black Face" seems to be a common problem for the "Bun Bun". I also have a moody one at home but unfortunately I am not as flexible to replace her. So I guess it really depends on how much risk can you affort? As for mine, as she can handle all her work pretty ok and hence the chance of me loosing the game by changing a new helper would be higher.
My previous maid was with us two years. During the two years, most told us that it's because she has worked as maid for long time (about 10 years for other families) and so she's like that. However, I finally made up my mind to replace her as I can't endure anymore. Also, she's taking advantage of your 'silence' and 'endure'.
don't expect she will improve. I think just change the maid. Though it's difficult to teach a new one as it takes time. But you will feel happier when you are home.