Summarize the following:
You can choose soft brush to avoid creating edges around your photo. Once it's done, exit Quick Mask mode and return to normal mode. You can see selection in your photo now. Then, go to Channel Panel and create new channel. Fill the selection with white. Go to Select > Deselect (or Ctrl+D) to deselect. Go back to your Layers Panel. The Lens Blur window will appear. Change the Depth Map. Click on the dropdown button and choose Alpha1 and hit OK button.

Summary:
Go to Quick Mask Mode (or press Q). Use Brush to select the areas you want to blur. Exit Quick Mask mode. Create new channel. Deselect. Go to Filter > Blur > Lens Blur. Save your file.