I literally just sewed my first cape today! The seams aren't perfect by any stretch, but black-on-black does hide a lot.
I've used the iron-on hem tape in the past, and it works pretty well. I'm not sure how strong it would be for something as large as a cape, but it's worth a shot.
I made mine in pleather so it's sort of heavy. The fabric was 55" across and I bought 6 yards to make a 6-panel cape. I'm shorter than 6 feet, however, so didn't really need the full 6 yards.