Short description of error
The 2.79 version does not have a code signature, while prior versions did (2.78c, 2.77). This causes macOS gatekeeper from allowing it to launch without manually overriding the security warning.
Exact steps for others to reproduce the error
Download Blender 2.79 on macOS 10.12.6.
Set system security settings to "Allow apps downloaded from App Store and identified developers" (the default setting).
Double click blender.app
A security warning window opens which says "blender.app can't be opened because it is from an unidentified developer". Version 2.78c did not trigger this warning.
Alternatively, right-click on blender.app and select "show package contents"
Open the "Contents" folder within blender.app.
An app with a valid code signature will have a folder named "_CodeSignature". This folder was present in 2.78c, but is missing in 2.79.