Vertical blinds

An elderly neighbour has such a blind across a sliding patio door. It is th e type with two controls; a chain, which alters the angle of the slats, and a cord loop which moves the blind over or away from the window. On inspection, I can see that the cord passes through a sliding block which supports the first slat. A knot in the cord, when the cord is pulled to op en the blind, easily pushes the block(and slat) along the track. There is n o knot (or other method) on the other side of the block by which the block would be pushed in the other direction, which means that the blind won't cl ose. I tied a slip-knot in the cord adjacent to the block. This seemed to have s ome of the desired effect, in that the block moved slightly in the correct direction, but the block repeatedly partially jammed so much so that I was reluctant to apply any more pressure in case I pulled whole blind off its f ixings. The lady did tell me(after about an hour) that she once found a piece of me tal on the floor under the blind, but she didn't know what it was and threw it away. Can anyone help, please?