Where was the reflecting surfaces when Muramasa fought Ichigo in Episode 233
There isn't actually any kind of reflective surface in episode 233. In sight of this, if we want to avoid the possibility of this being a plot hole there's only one possible explanation:


The creation and usage of the threads doesn't actually require any
kind of reflective surface, but in case there's a reflection of them
they can be disturbed through that.


On a side note, this might actually be a plot hole. The whole Zanpakuto arc is an anime-only filler arc, and filler arcs tend to be more likely to have or create plot holes.