I often wear belts on top of dresses and tops, and often with an open topper.
I somehow feel restricted by belted cardigans and blazers (in particular when I know I will have to be sitting for several hours), so I seldom wear that look.
My belts are medium-sized, and I like them to be slightly curvy or at least kind of soft. Braided belts, for instance, or stretchy fabric belts are generally able to follow the curvy lines of my body.
When wearing a belt on top of a relatively short top (and also if the outfit includes a relatively short open topper), I prefer my belt to be in the same colour as the top. However, if the outfit includes either a dress, another column of colour or a longer open topper, I feel OK wearing a belt in a contrasting colour.