I found black-hat content marketers: sockpuppet bloggers, fake Reddit/HN accts

Just go google "private blog networks" (pbn) or "link farm".

Good question. The thing that was news to me is that it's happening on very large blogs, not random sites and that it's being sold to and used by well-known startups. Like you, I expect sockpuppet accounts and random linkbuilding garbage, just not at this scale or with this much distribution.

On a large scale there is a big overlap between the general concept of operating a low-cost call center in a developing nation that speaks English (India, Pakistan, Bangladesh).

This has been taken to its most blackhat extent by the people who are running call centers for the fake "fix your PC it has viruses now, we are Microsoft support" scammers. Or even worse the fake Internal Revenue Service / Canada Revenue Agency type scam call centers.

The operating costs in monthly salary per human, and office rental, basic desktop PCs, electricity, telecom services are all very similar between the different grey/black hat business models. If you have humans talking to other humans by voice, you've got a bunch of $20 headsets with boom mics plugged directly into the desktop PCs, some SIP softphones, an asterisk setup, and a router/VPN connection back to a bunch of grey market SIP trunks. The voice part is obviously not necessary if it's just click workers.

Assume for a moment that your office and computer equipment is a sunk cost, and you've got a room full of dudes working a 6-day work week and paying them each $250 a month. They each need to bring in revenue of something like $10-11 per day to break even on the payroll. Obviously this "business model" is somewhat dependent upon finding a place that has a sufficiently large pool of low-wage, but moderately educated people who can drive desktop PCs, and a place to put your call center type environment in low cost commercial real estate. Thus my mention in another comment about Bangladesh.

Some organizations went into the gold farming market to hire people at the equivalent of $250 USD/month to repetitively perform tasks in MMORPG games and then sell the virtual currency/assets to people in the US/Canada/Europe with extra money to spend.

After that, they quickly discovered that it could be more lucrative to have one person run 30 reddit accounts (or twitter, whatever) and get paid to upvote stuff to the front page.

If one business model fails, you take the same office environment and temporarily convert the workers to doing another task. Or you have a mixture of tasks going on simultaneously for the click workers.

People with difficult accents or less than optimal phone skills, that cannot successfully scam a person and then transfer them to the higher-ranked "closer", are positioned for these click worker tasks.