ssmtp can only use SMTP. It seems like Sendinblue does support SMTP? Do you need to use the API then? If you need to use an HTTP based API for email, then you can bypass SMTP entirely and use a WP plugin.
After searching our forums, I was reminded that it might not be possible to use different smtp credentials unfortunately. See [Best practices] Different mail settings per site - #6 by strarsis
@strarsis you may remember this.
Related to this, I just found out sSMTP is unmaintained (has been for a while). msmtp is the replacement and would support this I believe. Also see https://discourse.roots.io/t/configure-different-smtp-users-per-wp-site/5716/4?u=swalkinshaw (though the full solution was never posted so I can’t be of too much help there).
I’ll add a backlog issue to Trellis to switch from ssmtp to msmtp.