Coincidentally, our Mentor just run this by us a few weeks ago at Tarot Classes.
I believe it symbolizes this queen's pretentious nature. She may appear calm and docile on the outside, but she's not above resorting to deception and trickery to get what she wants. It may also mean a dark secret, a perverse thought, an evil temptation she's harbouring. Or just basically, skeletons rattling in her closet.
Interestingly enough, our Mentor pointed out that this Queen's association with a black cat could very well indicate that she's a witch (literally or otherwise, I'm not sure), but that further adds to her untrustworthy and deceptive nature.