Unforunately, I think that will come down to what insurance companies think.
In my opinion, it comes down to whether or not the opened door crosses the marked line, but that's just me and how I would look at it if I had to decide.
If vehicle A opens a door, the door crosses the marked line and vehicle B hits it, then unless vehicle B had a "reasonable" opportunity to avoid hitting it, vehicle A is at fault.
If the door dosen't cross the marked line, then vehicle B is always at fault unless there is some extenuating circumstance that indicates otherwise.
But that's just my opinion.