To make photons more realistic, they should be affected by a black hole's gravity. So if photons were to go past a black hole they would curve and be "sucked in" by it. Also they should be able to disappear into the black hole instead of just bouncing off of it.
The black holes have no gravity, they simply induce negative air pressure. The Powder Toy has a very limited representation of gravity, which can basically be described as, "particle go down."