Hm, I don’t understand what you are trying to achieve here. In your blueprint select, you let the user select ONE category, but then in your template, you use toStructure() without having used a structure field 
Kirby does not know anything about the different options in your blueprint.
If you want to read the blueprint, this solution might help: Get label for select field value from Blueprint
Other than that, you can get an array of all categories that are used in all text files with the pluck() method.