This is especially true for vertical blinds, since they provide a way to make a room seem bigger. Parallel vertical lines create an optical illusion of height, just as horizontal lines do the opposite by compressing height. By causing your eyes to move up and down rather than sideways, vertical lines make the ceiling seem higher. Your head knows the room hasn't grown, but your eyes tell you it has. Just try it, and you'll be amazed at the illusion of space it gives the room.
These blinds can also work with other tall vertical lines in the room to create a harmonious, pleasing look. For instance, if you have tall plants, floor lamps or similar decorative elements in your living room, vertical lines on the windows are a logical choice. If you used horizontal blinds, they would break up the lines created by the tall objects, giving a disrupted, inharmonious appearance.
Many vertical blinds reach almost to the floor. These give a sense of drama and elegance, adding a bit of class to the room. Their large size provides a big area of colour that can really make a contribution to the decor. They can either match the other furnishings in the room or provide an interesting contrast to them, depending on your taste and the needs of the space.
Even the rays of light that shine through the blinds can be a sort of temporary decorative element in the living room. At certain times of the day, long beams of golden sunlight will shine across the room and create a pattern of vertical lines on the opposite wall. This can be a very pleasing look that will enhance the atmosphere of the living room for a while, and then fade to return the next day. You might even have a few objects of yellow or gold in the room to work with the colour of the light. Another good idea is to have a pattern of vertical elements on the wall being illuminated to enhance the warm glow of the sunlight falling across them. For example, a vertical mirror and a few tall pictures will look great with golden shafts of sunlight falling across them.
One possible objection to these ideas is that big, tall blinds on the window might be a little overpowering to the other decorative elements in the room. If you have this problem, try using vertical blinds with a printed pattern. This will soften the long lines, but some of the heightening effect will remain. In this way, you can give the illusion of a little more height but keep the effect under control.