I have eventually finished my first roman blind for dd's room, with a blackout lining. Instead of making pockets for the dowels and using the rings which you sew on, I was advised to use roman blind tape which does both of those jobs nicely. This obv needs to be sewed through both layers (fabric and blackout lining), which I did. I used a standard needle in new sewing machine for the weight of fabric (thick-ish cotton).
Have just had a final test of length of blind against window today, before get dh to attach hanging batten, and noticed all the horizontal sewing lines show up in bright sunshine as little holes through the full thickness of blind... ok, so not huge but somewhat peeved that not full blackout effect.
Used some very good instructions from t'internet - did I go wrong somewhere? Should I have stitched the tape to the lining only? And what can I do now - maybe attach a narrow strip of blackout lining over line where tape stitched, like a hood?
There are a number of ways to make up a roman blind. The best method with blackout lining is to make the channel for the rods by folding the fabric & lining to the back and sewing along the width. Blackout lining is black rubber with a coating, so any needle holes will show through; unlike fabric that would just part for the needle and close up after.
I wonder if you could put a dot of tippex from the back to fill up these little rows of holes? Never tried it myself but it may work.