Arq seems really good at supporting a broad variety of cloud providers though, so hopefully they'll add this too. I'm hesitant to use cloud backups generally; I've never seen an audit of how secure Arq's backup scheme is, for example (though it seems pretty simple - https://www.arqbackup.com/s3_data_format.txt). I've used CrashPlan a lot and basically take it on faith that it's secure. It's probably good enough for my use, given that I'm not storing state secrets or anything, but it's still a little unsettling to 'lose control' of one's data.
From Backblaze's point of view, I guess this is either smart (diversifying themselves–people can use other backup software if they like, and Backblaze still profits) or less smart (turning themselves into a commodity), but it seems like their software is still first rate, so I guess it'll work for them.