Can a player that doesn’t move considered to have prevented anyone from moving?
Yes. The obstructing player does NOT have to be moving. However the obstruction must prevent the defender from moving towards/with their opponent – this has nothing to do with where the disc is, so you can’t call a pick because you couldn’t get to the disc.
So if Black 2 is running in a straight line, it’s unlikely to be a pick. But if they turn right as soon as they go past Black 1, then it is very likely to be a pick.
Hope that helps.