Who put the blanket around Scouts shoulders and how does Atticus reach this conclusion? What is scouts reaction when she hears the information?

Jem didn't want Mr. Radley to know that Boo had left the house and provided the blanket. He told Atticus about the way the knot-hole was cemented in and didn't want to cause anymore problems for Boo. Scout was actually not that surprised to think of Boo as benevolent.