Whilst reviewing and discussing this patch for weltschmerz I found out that there actually exists a global property gtk-cursor-blink that is honoured by all applications that build on GTK.

I don't think I've ever seen this exposed in a settings dialog. Perhaps in the accessibility settings, I rarely venture there. In any case, you can put properties like these straight into ~/.config/gtk-3.0/settings.ini and GTK apps will pick them up.

For QT apps it seems you can set cursorFlashTime to a negative value in the aptly named ~/.config/Trolltech.conf but I could not get this to work with the only QT app that I use, quassel.

Not that I would want to, I do prefer a blinking cursor.