Xeno Accident

One question that's been bugging me for the last few days is, how did the Engineers know about the Xeno (from the Mural), because from what I can gather from events in the film the Xeno creation is an accident. Evidence: 1. The Black Goo is some kind of virus that turns the victim into an aggressive monster. Fiffeld and the Worms being the best examples. 2. The Engineers can't have known that procreation between an infected person and an uninfected person would create the Squid baby if they hadn't been to Earth to deliver it yet. Shaw and Holloway's intimate encounter would have been the first case, and therefore the Squid was the first of it's kind. 3. The Squid rapidly evolves into what can only be described as a pre-face hugger (or body hugger might be a better name) and delivers the Xeno seed into the Engineer. 4. Baby Xeno emerges from the Engineer. If the Engineers created the Murals then how did they know that the Xeno would emerge? Was it really them who made the Murals or was it the Black Goo reconfiguring itself to depict it's possible final form, in which case we are led to ask: is the Black Goo more intelligent than the engineers?