The Publisher Frontiers was - and, since there is no further development, still is - listed on Beall's List. Starting from scratch and with problems and therefore seen as predatory in the beginning by some researchers, Frontiers is now a reputable publisher with their journals listed in Scopus, Web of Science and the Journal Citation Report. The whole story is even included in Frontiers' Wikipedia article.
Regarding the question of what is a predatory publisher/journal and what is not, there is no clear answer or dividing line. There is a recent publication (still preprint) trying to gain further insight by analyzing the publishers/journals on black and white lists and comparing their characteristics.