You can use MailBluster with OpenCart using a 3rd party tool called Make (formerly known as Integromat).
The following link will help you with connectivity and integration using MailBluster: https://www.make.com/en/integrations/mailbluster.
Also, you can check out this detailed guide on How to connect MailBluster with Make (Integromat).
We also have a video tutorial on this which shows how to import Leads from Google Sheet to MailBluster using Make: