Do you think there will ever be a category in this game called Blindfolded Any%?

no because it would be hard to moderate (not everyone has a facecam and also it would be hard to verify if the facecam is consistent with the gameplay). Plus 'any% but' runs are generally less accepted

What would be difficult about beating the game blindfolded?

blindfolded runs are typically submitted to https://blindfoldedgaming.com/ or other leaderboards instead of the usual src page for the corresponding game, and blindfolded discussion often happens in the general blindfolded speedrunning discord (which you can find a link to on the site linked above)

a full any% run (without assist mode) has been done blindfolded with the use of a notched controller by katun, but only certain ILs have been done blindfolded without a notched controller afaik (cap, cascade, sand, lake, wooded, cloud, lost and snow).

as for the difficulties, there are many difficulties which come with playing blindfolded, and it is probably easier for you to just try a few kingdoms blindfolded to understand the difficulties instead of trying to convey them all through description

Some games on speedrun.com have a category on the leaderboard called blindfolded!!

Doesn't mean this one will. Odyssey already has a section on BFG.