"ont été blessé" = "have been injured" can one say just "ont blessé"?
No, that would be something like saying "How many people have casualty". "ont été" is the third person plural form of "être" in compound past, without the "été" it's the third person plural form of "avoir" in present tense.
Would "étaient blessés" be appropriate in the place of ont été blessés?
"ont blessé" is active and needs an object: "How many people have injured/hurt the child?" (could a French speaker please comment on whether it is ever actually used this way.)
"on été blessé" is passive "How many people have been injured?"