Many countries have passport blanks in every embassy. They're some of the most tightly controlled documents in the world given they could be used to print new, seemingly valid passports for anyone (if that country doesn't info share with the other, that is). No clue if they're being printed within the US in UK's situation, however.

USA has black passports as well ;)

https://py.usembassy.gov/wp-content/uploads/sites/274/2017/0...

My US wife’s passport and my UK one are a pretty similar colour. I actually miss the burgundy of the old UK one.

They are produced in the UK. There's something like a £10 fee for the delivery abroad, and it's sent with a typical fast service like FedEx.

They’re not printed in the US, but it’s totally possible to courier something from Europe to the US in 24 hours, maybe 48.

Yep, even from here in Finland regular UPS/DHL express services are normally 1 or 2 days to US (e.g. UPS Express Saver is 1 day to ~NY and 2 days elsewhere).

Though even postal items can arrive in a week as well, with luck.

Same day printing and DHL express would makes 1 week easily achievable.

What about the time needed to validate documents like birth certificates, etc?

If it's a renewal, presumably those documents have already been validated. One would need to validate the old passport being sent in but that should be it.

Delivered, accepted, sent for printing, and posted by courier from the UK, all with text messages to keep me informed of progress.

Mine was printed in another country and shipped 7200 miles to me in under a week for a $18 delivery fee.