Are there guidelines for necklines that go best under blazers?

I typically wear my blazers with a button up shirt, but when I try a solid color top/blouse, it looks ‘off’.... Is it because my tops are either crew or boat neck? Would a v-neck work better? (I have a giraffe neck and long head, so I don’t own any v-necks)

Oddly, crew neck graphic tees under them seem fine.