Virginia was never winnable for Sanders. He would've had to win the D.C. suburbs, which is implausible to say the least.
What really killed Sanders in the South wasn't just his losses with Blacks, it was the fact that he lost whites as well. In 2008, Clinton lost blacks by just as much against Obama, but she didn't get blown out of the water in the way that Sanders did because of her massive wins with Southern Whites (and Hispanics, where the existed).
Take a look at Texas:
Whites: Clinton +16
Blacks: Clinton +68
Hispanics: Clinton +42
A complete blowout with every demographic.
We can only speculate why Sanders did so terribly with southern whites, though a big clue is that he won whites in North Carolina. I wonder why? I have to imagine that if Sanders had actually invested in these states, he could have done at least somewhat better.
He did much better in NC than in VA for two reasons. One, as you mentioned, is investment. The other is that VA was open and NC was closed.
I think he probably made a wise choice. While it would be better in terms of the delegate math to win only Vermont but not get completely destroyed in Texas, Virginia, etc. the idiotic media does not cover it that way, and instead only cares about state wins. He was much better off trying to win 5/5 (he got 4/5) and have the media continue to pump him up as a real contender despite the fact that his path to the nomination essentially closed that day, rather than start the chorus of drop out calls "because he could only win his home state!" even if he actually ended up in a better position overall. Also, keep in mind he could not take MN/CO for granted, because at the time we had no idea how Sanders-friendly caucuses would be (though many suspected it, IA/NV created some skepticism) and internal polls, or any polls, don't help you gauge a caucus result very much.
Poll
