It's a large blue Indian Root Pills sign taking up the entire wall, and similar to the recently uncovered one just one block south on the same street. Here's a pic, taken from my post on this year's uncovering: http://findingtheradiobook.blogspot.com.au/2013/03/spectacular-ghost-sign-uncovering-in.html:
How do I know the hidden sign is there? A previous uncovering from 2001, photographed by Ian Harrison Hill. The photo was bought by the State Library of Victoria, which describes it thus:
Shows the north wall of 562 Nicholson Street, North Carlton, showing two painted signs promoting Dr. Morse's Indian Root Pills. Signs revealed by the demolition of the building at 566 Nicholson Street.
Note: it's actually in North Fitzroy, not North Carlton - that's the other side of the street.
I can't show the photo of the 2001 uncovering because it's under copyright. But you can see the photo online here:
The company behind Indian Root Pills must have had one helluva marketing budget in its heyday!