I wanted to take a look here at the imagery of the card to see if that gives us a clue to the cat. The Queen of Wands sits on a throne with lions on either side, also if you look closely the banner behind her also shows lions. The lion as far as I know is related to the astrological sign Leo and that rules the fifth house of the horoscope and is ruled I think by the Sun, a fire sign. The Queen of Wands is a very powerful leader that is connected to this fire sign- her scepter after all is a Sunflower and the black cat as her companion may well be a domestic representative of the lion - a little lion if you like!
Looking up in my Druid Animal Oracle about cats it has to say, "Cats bring us the ability to observe situations quietly without judgement before making decisions." Sounds like a lion laying in wait and watching its prey doesn't it.
It also says "the cat unites an awareness of the spirit-world with a highly developed sensuality." The Queen of Wands is probably the most sensual of the queens and she can be associated with using her intuition, as like all the Queens she is inward focusing and is Fire/Water.
Perhaps this little cat brings to our attention here that the Queen is a charismatic and sensual ruler, who is able to observe and make the appropriate decisions, by demonstrating a quiet self assurance, she pays attention to those inner feelings that guide her to be able to put in action what she feels is for the benefit of those around her.
From what I can see Waite wrote very little about this Queen in the The Pictorial Key to the Tarot by Arthur Edward Waite (1911) all he seems to have said (unless I missed anything) is:-
"The Wands throughout this suit are always in leaf, as it is a suit of life and animation. Emotionally and otherwise, the Queen’s personality corresponds to that of the King, but is more magnetic"
Again not seeming to do justice to Pixie's illustrations.
Just my little thoughts
~ HoneyBea ~