We've recently released a Blesta extension installer so users of composer can install Blesta extensions into their Blesta installation.
This makes it super easy to install open source Blesta extensions, or your own customer proprietary extensions using a private composer repository (such as satis).
This installer supports the following extensions:
Plugins
Modules
Merchant Gateways
Non-merchant Gateways
Invoice Templates
Reports
See the instructions for usage in the README.
Note that for Plugins, Modules and Gateways, the installer simply makes these extensions available in Blesta. You still need to complete installation in Blesta, like you would normally, for each of your companies.