The boy in the foreground needs no introduction. In addition to holding the crown, his turban has some gold threads running through it, suggesting some rank, making it none other than your truly, Charles II. The second one is more intriguing. It is not as though Charles II and Louise were involved in some menage-a-trois with some other male. But then it isn't too hard to know who else would have a secret interest in how the relationship between Charles and Louise fared. What cause there would be for portraying him as a black?
Who Does The Other Black Boy Represent?
Louise Kerouaille was a maid of Henrietta Anne, Charles' sister who was married to Louis XIV's brother Phillipe, and Charles got to know her on one of Henrietta's visits to England. He was smitten with her and couldn't lie. Henrietta died after a sudden illness in 1670 and that is when some plans were made to get Louise reacquainted with Charles. She came to England to serve as a lady-in-waiting (or whatever) to Charles's wife Catherine of Braganza, who found her more acceptable than some of Charles' other mistress who were openly disrespectful to her. The story is that Louise played hard to get - apparently 'French girls are not easy'. The rest as they say is history.
If any one had an interest in how the relationship with Charles and Louise developed, that would be Louis XIV, given how Louise was important to furthering France's relationship with England, especially after the death of Henrietta, Charles sister. This makes him the most likely candidate for the boy spying on from behind the pillar.
It really drives home the matter of Charles II being black complexioned and even better (or worse depending on your viewpoint) suggests the same thing about Louis XIV, especially as he was Charles' first cousin.
Which leads to another conclusion. The girl in the portrait of Louise Kerouaille by Pierre Mignard is probably an allusion to Henrietta, as she appears to patronize Louise via her offer of pearls and coral. Note that she already has her own pearl necklace round her neck, whereas Louise doesn't have one yet. Then again it could be Catherine of Braganza, but the coral causes me to lean in favour of Henrietta.