They get marked as inactive which they accept, and go back to their normal business of being a private limo service for the wealthy at $135/hour.

Uber Black in Washington D.C. is basically exclusively run by private limo operators driving Escalades. Owners of recent model $70,000+ cars are not your typical Uber driver, and do have other sources of income.

Almost every Uber black driver I've taken has handed me their card and proceeded to upsell me on their services outside of Uber.

That's not true with Uber Black. If you refuse an Uber X fare as an Uber Black driver, you're just marked as inactive temporarily (as you've declined a legitimate customer when there were no Uber Black customers). You can mark yourself as active again and take fares, but you are not forced to take Uber X fares as an Uber Black driver.

If it's as seamless as you describe it to be, it's not an issue. An external source here https://pando.com/2014/09/04/uber-continues-to-screw-its-par... claims "Uber drivers are required to maintain a minimum 80 percent ride acceptance rate to remain in the company’s good graces – the best performers exceed 97 percent, the company tells its drivers. Ignore too many inbound ride requests when you are the closest vehicle and the result could be some combination of reprimand/probation, lost bonus income, or, in extreme cases, deactivation of the driver’s account".

I don't have any personal experience driving, would be nice to have a clarification on that.

This only applies to drivers that decline fares in their current rate class. Standard Ubers, Uber XLs, etc. Uber Black drivers that decline UberX fares when there are no Uber Black customers are simply marked as inactive. They can set themselves as active again and accept fares again almost immediately. If they continue to do so, then there is a wait time simply because they're not offering the service they're contracted to do and they're creating a longer wait time for Uber customers.

In that case I agree this is a non-issue. I was going from the comment earlier in the reply chain that said they'd get kicked out.