I don't think there is anything inherently wrong with thumbs on black keys. Sometimes, there is just no other way. The concern is mainly that the thumb can't be controlled as accurately as the other fingers, and it is quite strong, so it may come out too loud. However, if you have good control, I don't see any reason not to use the thumb. The only exception is, if it misplaces the other fingers, so that the continuation is difficult.