I have both Blesta and WHMCS and both work very well provisioning CWP packages.
Having said that, at present i have had both programs installed on an independant Virtualmin system and use them to provision CWP, VestaCP and Virtualmin packages.
It should not make any difference where its installed, if you go to the knowledge base part of this website, they have a document on how to install it.
http://wiki.centos-webpanel.com/whmcs-module-for-cwp-apiI found that so long as you carefully setup the API access and ports, the WHMCS module works quite well.
Can i just say in passing, the difference in price between WHMCS and Blesta is only a few dollars per month. however, the difference in support, quality and capability is 10 times greater than that (WHMCS wins out hands down). Blesta actually is a rip off at its current price when compared with WHMCS...its not worth half what they charge for it. I have recently removed Blesta and no longer subscribe to it.
Reasons for my change...
1. WHMCS far far superior and better value for money (indeed the cost of 1 cheap cup of coffee per month difference)
2. WHMCS has far superior support and knowledge base
3. WHMCS immediately after install is very well configured in almost every area (Blesta has a lot of work to be done by user for email templates, invoice templates and so on)
4. WHMCS has a much better and more configurable client area and admin interface)
The BIG downside of WHMCS...everything is Ioncube protected. So you cant go into modules or any code to figure anything out when troubleshooting (a huge pain in the ass).
Overall though, WHMCS is currently the better option by a long way. Blesta is obviously a newer product and will improve over time, but not worth it right now for me.