Love Backblaze and it's my go-to backup recommendation, seeing this very late so not sure if you'll see my reply but a few thoughts:
>The current limit is 50 characters. There are reasons we don't let it get super long like 2,000 characters but we could easily double or triple the maximum length it if it was requested. Is there a number of characters you have in mind? I found a chart here: https://i.imgur.com/trR2u8g.jpg that says a randomly generated 17 character password would take 93 trillion years to crack.
Usually the reason people want longer "password" options is because they're using a diceware style pass phrase of 4-6 randomly generated words. It's not so much about ultimate security as many people having trouble memorizing decent fully random passwords. Key stretching and multifactor helps further of course. Longest normal English words are something like 21 characters before getting into niche science terms, so very conservatively 160 characters is probably going to fit even an extremely conservative edge case passphrase. That'd be 8x of some of the very longest words (unlikely to be randomly chosen) and match a 128-bit key which is plenty [0].
>* Backblaze also supports 2-factor authentication via SMS or Google Authenticator codes which personally I would highly recommend to customers also.*
I hope you'll consider adding (via Webauthn online or OS or other toolkits to the application) standalone hardware token support (like Yubikeys or Nitrokeys). Those are enormously more secure, convenient, and also have the bonus of eliminating another set of 3rd party dependencies. If you looked at those in the past and skipped them due to lack of device support worth looking again, every major platform should have coverage at this point.
----
0: Also, rather then thinking in terms of "years to crack" which inherently depends on the processor power thrown at it, might be better off considering in terms of combinations (or entropy). A standard symmetric key at this point is 256 bits, which means 2^256 combinations, but with stretching 128 bits is probably a very very safe seed for the password component. So that could be a "password" of [01] 128 characters long. That'd be roughly equivalent to [0-9] 38 characters long, or alphanumeric cased (26*2+10=62) 22 characters long. Current Oxford English dictionary edition I think has around 170000 words. If we figure a diceware program throws out ultra short, ultra long, and ultra weird words for convenience (whole point is something people can remember) so maybe it's cut down to the 70000 most common words >3 characters, that'd take 8. At a more typical 4-6 words, equivalent to maybe a 17 character cased alphanumeric password.