Negative pressure rise across blower
 

So I tried to match blower performance curve with CFD and it generally underpredicted results, however at the free flow condition with the highest flow rate the static pressure rise was negative in the simulation (this was the only data point with negative SP rise).

My BC's were volume flow inlet and static pressure outlet set to atmospheric. How does a negative pressure rise happen? I guess this is the CFD program basically saying, this flow rate you are specifying is "impossible" based on the assumptions being made in the model, therefore the only way to satisfy the volume flow rate BC is to increase the pressure at the outlet in order to achieve the correct pressure differential. All other data points at lower flow rates apparently can be satisfied in the simulation, therefore inlet pressure remains below atmospheric as the suction of blower is enough satisfy flow rate BC.

I imagine if I changed to total pressure inlet and flow rate outlet, the same result would have happened at free flow except this time outlet pressure would drop to negative (instead of inlet pressure climbing). I think of it like another imaginary blower is being placed at either inlet or outlet to aid in pulling/pushing the air through if the BC cannot be satisfied.

Is that interpretation more or less correct?

A negative pressure rise is a pressure drop and that is exactly what happens when a blower is not doing any blowing. It's basically a very lossy pipe. It is a windmill that is taking energy out of the flow to spin itself. If you want the pressure rise to be 0, then you need to attach a motor to spin the blower.