Generally, blinds are usually fitted within a window recess and are fitting so that they touch or sit just above the window sill.  This is to ensure that the blind can operate normally and blocks as much unwanted light as possible – if you fit within the recess the light blind is less than usually fitting outside onto the wall above for example.

However, circumstance or preference might mean that you need to fit ‘face-fix’ to the wall, above the window.  This could be the case if you want a blind for a window that would jut out too much for the window to open or close.  Exact size blinds are simply made without deductions, to the size specified.