For example, to be a 'Diabolist', you need to have 4 titles total, and a majority of them needs to be black; additionally, you can't have 2 black titles, or 3 black titles, because those would give you the titles of "Witch doctor" and "Death knight" (or their female counterparts). You can't have 1 black title either, because then you wouldn't have a majority This means you need to have exactly 4 black titles. There are only 3 black titles. (per gender)
I haven't checked all the combinations, so perhaps there are others that are also impossible to get.