Hmm, I couldn't find any merchants selling boots blueprint after a quick check.
Looks like it might be missing from static merchant store inventories.
About rarity, I think the system works works like this.
- items can appear in random loot table, which means they can be found randomly pretty much anywhere!
- they can also be static in some stores, which means some trader will always sell it
- items have hidden item level. this limits where it can appear (eg, a level 5 area/enemy can't drop level 6 items)
- and visible itemquality (see item's name: low, normal, high, rarity, artifact). this affects how rarely they appear as random loot.
Not quite how it works, but close enough. Two significant things to keep in mind are:
- loot is not completely random; depending on what you're looting and where you access only a certain subset of the loot table
- the color coded item quality doesn't correspond directly to the item rarity in the loot table, it's only meant to give you a general idea; actual rarity factors are stored in the loot table
