A blow filling fountain pen looks like a standard dropper filled fountain pen. There is no visible filling mechanism. When you remove the threaded section, there is a sac attached to the section. Your other clue that the pen is a blow filler will be a hole in the end of the barrel. To fill the pen, put the nib in ink, seal your mouth on the end of the pen and blow hard. The pneumatic pressure will collapse the sac. Now take you mouth off that filthy pen! The pneumatic pressure that collapsed the sac is released, and the sac fills. Simple. Gross.
You could also use a separate bulb on the end of the pen to do the same thing. But at that point, why bother? You would be better off buying a better filling system or using a dropper filler.
The most famous example is Crocker Ink Tite (maker of the other “hatchet filler” with the lever attached to a barrel end knob).