Hey,
I attached a pic from the back of your powered Blackstar bass cab.
Theoretically you are right, using it with a modeller or preamp is the point of an active cab :) so there has to be a way to make this work for you.
I suppose you are using the left combo input (in the "active" section)? If you did plug it into the passive input, there will be nothing happening, as you describe.
I hope this helped,
Jakob