Well, first, we need a standard, a definition, of Football Blue Blood, and that is not all that easy to nail down. I think it comes from a combination of things that have been manifested over time, with the stress on this century and probably back to the 1970s. Pitt, for example, has a bunch of National Championships before WW2, and long averaged more fans per game than PSU. But it would be stupid to argue that Pitt is a Blue Blood now, or any time since Foge Fazio ruined a good thing for keeps.
National Championships over the past 50 years matter. Major conference championships matter. Final rankings in the Top 10 and Top 25 matter. Bowl appearances and bowl wins matter. Average attendance matters, as does TV drawing power. Who your blood-and-guts rivals are matters.
Blue Bloods elevate non-Blue Bloods in the conferences. That is the reason you want as many of them as you can get.
I agree that the SEC and the Big Ten are full of chicken plop when they assert that they have no need to play really tough OOC schedules because their leagues are so filled with Blue Bloods and so tough that they already play killer schedules. I think every team that gets into the playoff should be required to have at least 1 P4 OOC win. Such a rule would force almost all SEC and BT teams to play 2 P4 OOC games per season. And that would be great for ACC TV exposure and money.
And it is money, not National Championships, that the ACC now needs a lot more of.