I know this has been posted quite long time ago and I'm here to bump it up
Personally, I think affiliates feature is essential for a billing system as that helps increasing sales, finding new customers.
Blesta truly need it own affiliate system. I know there is https://www.idevdirect.com/ plugin but look:
1. Its license cost 2x more than Blesta alone.
2. Their interface is buggy, no way that would match with Blesta. (main one)
3. Missing proper functions such as automatic payment for affiliates, recurring commission, sales tracking for affiliates, ...
4. Blesta won't be comparable with WHMCS if we miss this features, I know plenty of providers that went for WHMCS just because of their sale tracking + affiliate system.
I think this should marked as high priority instead