Thanks for the replies Amethyst Deceiver and s7o.
you made the edit in the nohelmet esp? and loading it last didnt update it in game?
Yes, I tried it both as a single esp, with only the edits you mentioned and then when I tried to add it to No Helmet.esp I noticed he had already done your suggestion in his mod, but it wasn't working either no mater if it was Load Ordered by BOSS or loaded last.
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just find any hoodless robe, make a new texture set with the blue robe textures, assign this to a copy of the hoodless robe, done.
or, if you want it to be a replacer for the original robe, assign the hoodless-model to the base item instead of the original one and retexture this.
edit: oh, should have scrolled down before posting

Yes, it's really odd thought that would have fixed it too.
First I thought the reason the change wasn't showing up, was some robes had already spawned. Possibly only a NPC I haven't met yet would be effected but I've had No Helmet active with these changes since the game was started over 70 hours of gameplay.