@ecks What other devices and controllers do you use the USB? What is power rating on the fan that you will be using? Does it have any labels on it?
There is no label saying anything about the power rating on it which you can see in the images and like i said USB wise for cords i use it for my game controller and phone to recharge the only thing i have left if an old micro usb charger but it is not even 5 volts it is more like 2 or 3 volts instead (was made for a super old flip phone)
@Wizardling I can't speak to the general design choice for an outward blowing fan in most consumer electronics blow outward. This won't prevent dust accumulation because what goes out must come in somewhere else. This is usually remedied with air filters of some kind.
I use an inward blower and it seems to be working great. My Pi3 generally runs about 20 degrees cooler with the fan. I also use a heatsink applied with arctic silver thermal adhesive so the cooling is quite efficient. I'm not concerned about dust buildup because I just use it for gaming sessions that last no more than a couple hours once or twice a week so I imagine to get enough dust buildup to cause a problem will take years.