@rooftopjaxx ah ..this isn't the actual blind that's not working, just a similar one that was closer to hand. I don't think there's a spring mech in these.
It's very weird that the cord turns fine when I pull on the bottom of the blind to close it, but it feels completely stuck when I pull on the cord itself. No problem to open it using the cord. I'm generally fairly practical, but this has me bamboozled.