only one partition mate, the blue bit is used space and the white is un-used space on the existing partition. Also it only has one volume (Macintosh HD) on the left under your main HDD.
Hope this helps mate.
Edit: misread, I noticed you said 500GB HDD. obviously Disk utility is reporting different only claims 250GB, what is obvious that there is no other partitions, but I've no idea why Disk Utility says you have a smaller Disk.
You sure your model had the 500GB when you purchased?