I am using B2 now - I started using it before they added the encrypted buckets and am using restic to encrypt everything. It's nice because I don't really have to trust Blackblaze at this point aside from them not losing my data.
I've since additionally turned on encryption on my buckets, but as far as I know they store the key for you, so in terms of privacy it's not the best.
+1 for restic. I've been using it for four years now and have never encountered an issue, including during my yearly restore practice run.
As far as B2 bucket encryption is concerned, I wouldn't trust it as far as I can throw it. Quite honestly, it could just be a fancy checkbox on their website without any actual encryption, and we wouldn't be able to tell. Either way, a compromise of Backblaze would put your data at risk.