Use blob local instead of instance.

It is open to discussion which is the better style, to use instance variables `@blob` on partials or only locals, but mixing both like here is definitely not the way to go.

http://stackoverflow.com/questions/2503838/rails-should-partials-be-aware-of-instance-variables

Modified to a local since it is the only occurrence of `@blob`, and there are many other `blob` in the same file.

Other variables however are predominantly instance.