I had a similar issue running cgminer on Windows 8.1.  Not sure it's the same thing you're seeing, but this is what fixed it for me:

1. Disconnect your GridSeed Blade USB from your computer.
2. Run cgminer (or in your case cpuminer).
3. Plug the USB back in.
4. Wait a few seconds and you should start to see the usual accepted/rejected lines.

I'm assuming you did the Zadig WinUSB driver install step before all this.