I like the air idea better than my food color MO. But instead of cutting pipe, you could first trying fashioning a cover of sorts over the black pump's basket chamber with duct tape. Use a good number of layers, and leave the center open, or cut out a hole the size of your blower. Blow the air through that and see what happens. That'll either work or come apart. Then you can cut pipe.
Blowing that way up the return line you will be trying to blow through the impeller. I think the impeller will give a big restriction to much airflow. You can give it a try but I think you will need to blow into an unrestricted pipe.