Actually, the blank walls could be used for murals. I would love to see more and more of those around downtown - in addition to the ones that are already there.
There's a section in Kuala Lumpur along the river that has concrete embankments that they've let a lot of graffiti artists take squares and go to town on them. There are so many different murals that are all so unique - I think it's one of my favorite areas of that city because it truly is art in most cases. I don't have any of my own photos on my computer at the moment, but here's a link I just found with some great shots:
http://www.anothertraveler.com/graff...-kuala-lumpur/
Tagging is a separate thing altogether - not art, in my opinion, just rude and damaging to the neighborhood, much less someone's property.