I just got a Blue Snowball iCE, plugged it in and my Mac recognized it fine. In System Preferences > Sound > Input, Blue Snowball shows up and is selected and I see the input level meter moving in response to sound. There are no other mics plugged in.
In Garage Band > Preferences > Audio/MIDI > Input Device set to Blue Snowball and then record and no sound is recorded and the level meter doesn't show any audio input. When I switch to Built In as the source I also get nothing.
I tried plugging the mic into each of the two USB ports with no change.
I also opened the Audio MIDI Setup App and tried turning the volume for the Blue Snowball all the way down and back up which I saw as a recommendation somewhere.
I downloaded Audacity (2.3.0) and it is able to record fine from either the Snowball or the internal mic.
MacBook Pro (Retina, 15-inch, Early 2013)
The Blue mic works fine on a MBP 13" Late 2013 running the same version of macOS and GarageBand.