Directive Blankets can be Reshaped, however the process is NOT intutive.
Select a mid point handle in the existing blanket with LMB, then TAP the insert key and drag.
Do Not hold down the Insert Key. Hold Mouse Left Click Down on a Handle then Tap the Insert Key Once at each handle, after Tapping the Insert Key Drag the Blanket.
Or redraw the blanket using a series of LMB clicks
The LMB click points were made at each inside and outside corner.
That's it !