Blue Dragonhide

Blacksmiths hate this one simple trick. They spend their lives mastering interlocking chains and plates, only for dragons to be born with them. A third of the blacksmith's punters are now gone, hunting for dragons rather than dropping some coin at their forges. It's enough to make a blacksmith sob into their anvil. 

Blue dragons hate that one simple trick too, of course. Their venn diagram overlaps with the blacksmith's: they'd rather people got their armours from somewhere other than dragons.