How do I fix the twist mechanism on a blind?
I would repair this by fitting a short piece of aluminum, copper or brass tubing over the nubbin of the shaft. This can be glued in place using epoxy. Before installation the end of the tubing can be worked to make a new end to engage the wand. The simplest (working with copper is especially nice) way would be to squeeze the end of the tubing flat. Then trim the corners of the flattened section to a rounded end profile. Finally make a hole in the end that can take a ring linkage from the adjustment wand.

I was going to draw a picture of the idea but it was faster to just make a sample of the proposal. Just choose the tubing with an inner diameter that is close to the size of the nubbin. The hole for the linkage gets drilled through the flattened end. Before trying to apply the epoxy in the tube do use a Q-Tip dipped in isopropyl alcohol to clean away any possible oil film left from the manufacturing of the tubing.