Make barrier blocks visible

It's a particle

If you change the particles in settings to All, it will shown barrier blocks when holding a barrier.

One thing you could do to make the barriers even more visible would be to create the maze with an unused block ie. dirt. Once finished, use /fill <x> <y> <z> <dx> <dy> <dz> minecraft:barrier 0 replace minecraft:dirt to turn it all into barrier blocks.