Just wondering if it's a skinstudio or windowblinds setting that is causing it.
Neither, it is the image and the way it was created, the gap at both ends of the taskbar is by design for that skin, there are many skins and skinners creating WindowBlinds with the taskbar designed this way right now.
You have 2 options, either find a skin that you like that has taskbar the way you want (which there are many, many of) or if you want to use skin you are posting about you will need to edit the taskbar image to make it go across screen.