Another problem with these animal death-match questions is that hunters are always more cautious than they need to be. Even if you can attack a full-size, healthy, aware blue whale head-on, why in the heck would you? There are plenty of smaller, sicker, unaware victims who are far less likely to hurt you back. A predator can’t afford to be injured; even a 10% chance of being hurt means you’re dead in less than a year.
Sharks are a perfect example of this. They are attracted by blood, and by odd vibrations in the water that suggest an injured fish. Even then, they’ll circle the target and attack from below or behind whenever possible.
So, while I don’t doubt that the megalodon could have killed the blue whale, it would have to be one desperate shark with no other options to even try. They’re just not interested in a fair fight.