there are other addictive substances other than black potions in the world. The black potion is just the only poison that works on dragons so they specifically used that on everyone around the kings of the royal line for generations.
That being said, a regas that they were sending to the king would be a target for that, but it was Mataaz who gave Blondie the meat that is a local delicacy only found in the region he and the first commander grew up in. That means that the Duke couldn't have known about it. Even if he DID give Mataaz the black potion to give the regas before they learned whether or not the king would even take a liking to him, the jerky strips that Mataaz was snacking on were all bundled together in the same package. He would be poisoning himself too if he put the black potion on any one of them.
What I'm guessing happened is that the jerky has some strong region drug in it that Mataaz has built immunity to. Either that or he's able to fuel his addiction to it give his current high status and wealth which would also explain why he looks like he's on the verge of become a full blown coke addict all the time. Either way, I thought he fell into Norhux's trap during the carriage ride with Blondie and during the resulting fire that killed all the other regases, but it looks like he was laying the trap himself the whole time. He intentionally took the loss in exchange for getting Blondie addicted and probably intentionally let that info slip in the carriage ride too.