It would be nice if it were that easy... console.log() only outputs client-side stuff to the client; however, since you're working on the server-side of things, you won't see anything pop up in the browser console (rather the server console, probably your terminal).
There is a way around this thanks to the work of others, for instance:
https://github.com/ethanl/connect-browser-logger
This will basically hoist those server side logs out to the client for you to see. If you do a Google, you'll find a slew of other solutions (some with the ability to set breakpoints, step through code, etc).
Not shabby!
Edit: Christ, I just realized you wanted the logging specifically IN your gruntfile. That's a bit of a different story, but it still should work for you!