Short answer up front, you probably haven't set up your Hidden Artes properly and the game is just picking what artes it thinks is best right now, who knows what criteria that follows though. I believe blue denotes inputs that will result in the game picking an arte automatically.
Long answer: It's been awhile, but if I remember right, there are two artes menus. Your Martial Artes just slot in automatically and as you level up you simply get longer combos. Random example screenshot. The Hidden Artes work a bit differently, in that you can set up to five different artes whether you are neutral or pressing left/right/up/down on the left stick.
Now, this menu is a little weird at first. When you put a Hidden Arte into the left or right slot, or the up or down slot, it will clone itself into its opposite slot by default, so left or right performs one arte and up/down performs another, an arrow will connect them and the clone is a darker colour. Now, despite this, you can still set up four artes across all four directions, you just need to select one of the cloned slots and choose a different arte to go there, but during the early game you didn't have enough Hidden Artes to do this.
Now your problem I think involves the default neutral slot; slots that are not assigned a Hidden Arte will default to "Do Whatever", letting the game decide which artes you should be using at a given time. I don't expect these decisions to be very intelligent, probably just based on raw damage or counter to the current enemy type that you're fighting, rather than one that flows well with your other artes or ones that let you defend yourself or set up combos. You can totally overwrite the "Do Whatever" slot with a fifth arte.
Of course, also make sure to visit the Battle Tactics menu and the system books to learn about Blasts, because Banish Blasts are a critical aspect of Armatization. There's also a defensive/healing move mapped to the Blast button during Armatization, but I reckon you probably know about Blasts from the tutorial prompts.
Dr Pepper. It's an intellectual drink, for the chosen ones.