Maybe "pegando um bronzeado" will suffice, I think that in the sentence she meant negro-tan. Grooving would mean "an pleasurable experience" (think of nice/cute, etc), fat chances here.
Notice that certainly this is a conversation between two people that by convention might think the word negro as dark (so - pegando um bronzeado bem bronzeado, não um bronzeadinho). She could express that by "developin a dark tan" etc, a more "political correct" sentence, since today many people avoid using the word negro in certain contexts.
Provided that this NEG thingun meant that, of course. With English (and these days) one never knows!