During my many years selling window treatments, I learned to avoid the term ‘black-out’. Black-out suggests complete darkness and no product by itself can achieve this. There will always be light leakage through the ‘halo-effect’ where light passes around the edges of a shade or through the spaces between blind slats. Some ‘black-out’ roman shades even allow ‘pin-hole’ light to pass where the liner attaches to the face fabric.
A more accurate term is ‘room-darkening’. I would point out to customers that, while the fabric itself might block light completely, some leakage should still be expected.
If total darkness is requested, you may need side panels and a top treatment to get closer to the goal.