I haven't made Roman Blinds, but have made a number of curtains and Austrian Blinds. I have had two houses where I have made window coverings for every single window.
Background
Windows are heated up by the sun and can turn into radiant heaters very quickly in summer, so anything on the exterior that blocks the sun from hitting the window will reduce overheating far more than curtains or blinds on the inside.
Windows are the major cause of heat loss/heat gain because they are much less insulated than walls. Air inside a room circulates from top to bottom, with the air either cooling or warming as it circulates past the window. The circulation is not stopped by curtains unless they enclose the window at the top, bottom and sides. This is why I have pelmets and my curtains are all floor to ceiling and go past the window, to create a seal there too. Once you have that seal, the room has two sets of air circulation - within the room, and between the window and the curtain.
Responses
Blackout gives a good seal, but the thermal lining provides a better insulating layer. Thermal lining is sometimes thicker and fluffier, which may not suit roman blinds (but you could always make them three layers with the thermal lining in the middle.
I don't really like roman blinds for insulation because the sides don't have an air gap as they tend to V. Roman blinds would be domewhat difficult to sew because you have an enormous amount of material that you are trying to put through the throat of your sewing machine when you are sewing the middle parts. My Austrian Blinds were fully lined with blackout (thermal lining wasn't available so I didn't use it), and were used every day. You need a lot of extra space at the top of lined blinds because the material is much more bulky - otherwise it will block a lot of the window.
Similarly, lined curtains take up more of the side of the window than unlined curtains do, so it is worth extending the curtain rods a bit further onto the wall.
Pelmets are useless if they don't provide a seal above the curtain and in front of the curtain.