Somehow, at least to me, they would find more use if they belonged to the bottom category.
The Side Blade
Compare with Bottom Accessory Cat Tailand Wolver Tail, and Back Accessories Vita Kit, Wings or Dragon Wings.
It doesn't really fit with either, but when in doubt I'd see it more as a bottom item.
Also, there are fewer Bottom Items than Back ones, moving them to bottom would make more space for other expensive things.
Only problem I see, would be Items that already have a parrying Blade and another bottom item - through leaving them as they are wouldn't hurt anyone either.
It's just something that I wanted to have said, through it's nothing world changing.
All back/bottom accessories used to be in the same category.
The "bottom" accessory was made specifically for tails, so that people didn't have to sacrifice wings or vitakits or whatever for their tails. It was also changed so that sets like Chroma sets could have a back accessory so long as it didn't conflict with the sets natural tail, as Chroma sets used to couldn't have any accessories at all on their backs, whereas now they can thanks to the change.
There was, and is, no reason to separate the blades' category.