What problem does this address?

For blocks which have block styles registered, once a style is selected in the block settings sidebar, either by being set as the default style or by manually selecting it, it's not possible to remove the style. If there is more than one style available for a block, whichever style is last selected cannot be removed, i.e., there must be a style selected once any style is selected or assigned by default.

While it is possible to remove a block style's corresponding class in the Advanced→Additional CSS class(es) field, this is not an ideal primary solution, since that field isn't necessarily obvious for users. It would be better to locate this functionality in the same interface where the styles are selected.

What is your proposed solution?

Add "reset" functionality similar to that of other block settings such as spacing or text, with a label such as "remove style".