Yes, I made sure that my initial stocks were mixed sex pure bred blue tilapia so that I could keep breeding and growing more.

In-breeding can become problematic if left unchecked, so you either need to actively manage the gene pool (by controlled breeding and culling) or passively manage it (by occasionally injecting new genetic stock into your population from other fish farms). The best approach is to do both active and passive management, imo.

Nature's pretty resilient, though: I doubt you'd have much trouble producing all the fish you'd need for decades even if you start with a few dozen of one strain and completely ignore broodstock management of any kind. You probably wouldn't grow world-class trophy fish that way, but you'd still be well fed.