Mac OS. To put it simply, Nothing Happens when I try to use the Blur Brush Tool. I have just enough acumen to use the clone stamp. I know which other key I must use to activate it. When I experiment with the usual four lower left keys, Nothing Happens. Some simple critical silly little step or setting is wrong and I have no idea what it is. Opacity? Background? Locked layer? Someone else had this problem I see, and suggestions to help did nothing for me as I tried to blend away the striations you see in in the screenshot. If you try to help me, please—just tell me what the click sequence is to wake this thing up. Well-intentioned analyses about layers and pixels will just confuse me.