Let me correct the French sentence first: 'Le nombre de chapeaux noirs diminuait à vue d'oeil, et la plupart semblaient avoir perdu de leur hauteur normale'. I will assume that the last two words refer to people's height, which should be 'taille'. 'Normale' doesn't sound right here either, but at least the grammar is fine. 'The number of black hats was rapidly decreasing, and most seemed to have lost part of their usual height' (they grew smaller?)

I am not an authority in French, but here's my try since no one gave a full answer yet. "The number of black hats was noticeably decreasing, and most of them seemed to have diminished in height."

"à vue d'oeil" can also be translated as "visibly" or " before our very eyes" -----> The number of black hats was visibly decreasing,... The number of black hats was decreasing before our very eyes,...