One solution would be that they have to carry the portal or build it there. My initial thought would be that they can teleport wherever they want, but it's a very complicated/slow/dangerous process, they might be hard to get because they live underground but they expand very slowly. They might be blind, but that doesn't mean they could invent something that translates the universe to them. Like how we can use equipment to check magnetic fields, UV, sound waves, etc.. They can see, but not using light.