Have you taken a look at the Shared Login plugin for Blesta?
I would have used that plugin to allow for SSO to Blesta, or integrate OAuth support with something like this Wordpress plugin. This will get you to log in to Wordpress via Blesta. From there, you can use Blesta's API to retrieve information on the client, like whether they have any services, and redefine their role in Wordpress.
If Blesta needs to push notifications to Wordpress, you can create a Blesta plugin that listens to certain events and makes API requests to the Wordpress installation to perform necessary actions.
Regarding user authentication passwords, these will never be decryptable in Blesta and they're not supposed to be encrypted anyway for security reasons.