Goldman and others got in trouble for selling access to the supposedly secret orders in their dark pools back in the day.

They were fined 800k I think?

I believe dark pools started growing post flashboys when people realising what hft meant and wanted an escape (see comment below). But as they got big they became a target in and of themselves (ecology in action I guess).

I worked in algo trading at the time (not hft, but anti hft)