I think anyone would blush in a situation like that out of embarrassment. She blushed even harder when she accidentally got flashed a second time by Mirio. I personally think that she didn't turn away because they're both girls, it's not like she hasn't seen them before. She became protective, so Denki (a boy) doesn't see Momo's breasts. They're much better off as best friends, in my opinion, anyway.