Blinds are available in indoor and outdoor forms, and they both have their own reasons for being used. But put them together — outdoor blinds on the verandah, indoor blinds over the windows looking out onto the verandah — and you have a nice system for helping to keep your home cooler. You may still have to use air conditioning in summer, but overall, you'll have to use less.
Outdoor - Blocks Light and Its Heat Before They Reach Your Home
One of the issues with indoor blinds is that light hitting the window glass can still transfer heat into your home, just not as much as it could when the blinds were open. The light heats up the glass, the glass transfers that heat to the air molecules between the glass and the blinds, and ventilation allows that heated air to travel into the room. With outdoor blinds in front of the house, you prevent that light from reaching the outside of the window. The interior of your home is less affected by the heat from the light and thus remains cooler.
Indoor - Filters out That Annoying "Side" Sunlight
No matter what you do, it seems, there are always times when the direct sunlight you want to block shines right through the small, open area to the side of the blinds. Many outdoor blind models are motorised and connected to the sides and top of the area they cover, thus blocking that light. But non-motorised blinds may be open on the sides and let sunlight shine through those gaps on the sides. When that happens, closing the indoor blinds can help block that light from heating up much of the interior of your home.
Both Allow for More Customisation When Opening for Filtered Light
Indoor blinds allow you to alter the amount of sunlight coming in and let you filter the light to make a room brighter but not bathed in direct light. With outdoor blinds blocking most (if not all) of the direct light, you can open the indoor blinds a little more for light without heating up your home. That saves on energy bills as you don't have to turn on lights during the day in order to see anything in the room.
For windows and doors that look out onto a covered patio or verandah, having both outdoor blinds on the verandah and indoor blinds covering your windows can give you a double layer of protection.