I can't see the governor of California being a great choice, people are leaving California in droves, it's big cities are becoming unlivable.
The Democrats should have been thinking about who could run this time for years. Harris was never realistic, she is known to be an incompetent bully with an unlikable personality.
Trump, and the Republicans, have been eating into the Democrats core voting blocs more and more. They lost many Catholics, they've lost the (huge) Hispanic vote as a reliable block, and the young black male vote seems to be following - none of this should be surprising as this groups are typically more conservative than the average American. They need to find some who will appeal to centrists.
I don't think being black or a woman is particularly a barrier - there are lots of both holding elected offices in the US. Presidential nominations tend to weight toward the wealthy, so more are white as they are statistically more likely to be well off, but that's a correlation.