Using the blur tool - no visible change

[Mark30Croton]

Hopefully this isn't just me being dense.  
i have a photograph of a rock wall 

the image goes left to right. 
i want to blur half the wall

I select the blur tool and paint the area but nothing gets blurred. 
what am I doing wrong?

thanks 

[Mark30Croton]

I had done that earlier but I understand