I just got a Blue Snowball iCE, plugged it in and my Mac recognized it well. In System Preferences> Sound> Input, Blue Snowball is displayed and selected, and I can see that the input meter is moving in response to the sound. There are no other microphones connected.
Under Garage Band> Preferences> Audio / MIDI> Input Device, set Blue Snowball and record, and no sound will be recorded. So I switch to Built In as a source and nothing.
I have tried both USB ports.
I've downloaded Audacity (2.3.0) and I'm recording well from both microphones.
MacBook Pro (Retina, 15 inches, early 2013)
The Blue Mic works well with an MBP 13 "running at the end of 2013 with the same version of GarageBand.