Tenantos is an application that provides everything to automate server provisioning and management for cloud providers and resellers. It integrates with both VMware and Proxmox.
Billing integration with WHMCS is already available so it wouldn't be much build a similar integration with Blesta.

API Documentation URL:
https://api.tenantos.com/docs/

To get an idea of the billing integration here is the WHMCS documentation: https://documentation.tenantos.com/billing-whmcs/