What's the best way to create a wood elbow?
I hate to have to rain on your parade but there is a reason you do not find ready made wooden curtain rods as you describe. At a span of 76 inches even the straight part of a dowel rod that is less than an inch in diameter will sag over time. Add to that the extra weight of black out curtains and the straight part of the rod will be even less effective. This is the reason that the usual straight wood curtain rods that you find will be at least 1.5 inches in diameter if not even 2 inches. And at your span such rods still need to be supported in the middle which leads to ring hung curtains having to be center opening toward two sides.   

You have the additional consideration that ring hung curtains on an open rod leave considerable space at the top for light to come in negating the effectiveness of the black out considerations and your stated objective to want to have the curtains seal off light at the side returns to the wall. Top sealing for light shielding can be done with a box type valance structure this hides the curtain rod assembly so there is little point to try to design the curtain support to the aesthetic of the ring hung style as you pictured in your question. And if you try to avoid the look of the box and use a double rod structure with the outer one hanging a valance it has to fit tight up to the ceiling and that would hide the other rod the curtain hangs from....so it just as well may be a ready made metal one even avoiding the the ring hung style.