Welcome to TRF, Countessa.
Though Princess Angela undoubtably lives up to all the qualifications that you are giving her, I highly doubt that her lack of recognition is due to the fact that she is afro-american. It has more to do that she herself and her husband keep out of the publicity as much as possible and live a rather private life in the US, focussing on their children and their respective carreers.
The size of the pirncipality matters here too, it has only 40.000 inhabitants. The Princely family of Liechtenstein is the largest royal family around, so that gives most of them very little to do with their background. Consequently the European press takes less interest in anything to do with this family then with the other RF. So Angela, Maria and all the other xx(x) princesses of Liechtenstein will only sporadically apear in magazines.
Angela's sister-in-law Princess Marie (born Countess Kanolky) is hardly more visible or recognised then Princess Angela. Why? Because she leads a life away from the press too and like Angela she doesn't perform to many (if any) official duties either.
Hereditairy Princess Sophie (nee Dss in and Pss of Bavaria) is somewhat more 'visible' , as her husband already took over some tasks of the ever-grumpy Prince Hans-Adam.
There was a controversy about the marriage, that is true, just as there was a controversy when Max's sister Tatiana married the untitled German nobleman Philip von Lattorf. This has nothing to do with the colour of Angela's skin, but with the fact that she isn' t a noble or royal by birth.
Some members of the Princely family of Liechtenstein are more visible but that has mainly to do with them visiting certain society parties in Paris. Prince Phillip and Princess Isabelle (brother and sister-in-law) come to mind, as does Princess Barbara of Yougoslavia who was born as a Princess of Liechtenstein. Prince Nicolaus and Princess Margaretha naturally show up at many royal celebrations throughout Europe as she is the sister and daughter of a Grand Duke of Luxembourg. Princess Marie of France, also married to a Liechtenstein Prince also occassionally shows up here and there, though much less often.