I like the idea of an Ability Block having a cosmetic effect on magic, but not a harsh restrictive effect. You can use craft magic to create things as normal with the regular roll to see if the item is functional. But it should look kinda bad and have some wibbly-wobbly aspects that are amusing and minor. You can, say, magically create a tower even though you have no grasp of construction. There is always a draft, the stairs are crooked, the front door is cockeyed, and the floor is slanted in one part. But it will endure and serve as normal otherwise with no mechanical effect. If you are blocked from Combat Abilities, your combat spells function as normal, they just look clumsy.
I personal also like the idea if you use magic to get past a Ability Block the result have some minor flaws. Beside crafting magic appears to be flawed compared to normal crafting as it is special mentioned in the books that it can't be used for Verditius craft magic " The mysteries of the Verditius art cannot be replicated with simple craft magic.".
Taking a flaw means that a player wants that flaw to be relevant to the game. As such, taking "Ability Block (craft)" means that either the player is setting up a character for whom this block is relevant, or that they are asking the Storyteller to make it relevant.
(me, as Storyteller) - "man, if only you knew how to brew beer, you could have such a great conversation with this Virditius, who would probably be happy to share his secret of vis-laden alcoholic beverages."
(me, as Storyteller) - "what do you mean, your character can't figure out how to size a shoe? The apprentice brownie is offended at your inattention to his craft. Looks like you're going to have to go to the local farie court to apologize!"
(me, as Storyteller) - "the local noble is talking about something regarding wall-building and his castle, but you couldn't be bothered to listen. And for some reason, he's really angry right now."
In other words: if you don't bring it into play, I will bring it into play - because you asked me to.
Note that I wouldn't treat a minor general flaw as a major story flaw - rather, I try to get flaws to overlap. So the above examples would be regular plot Hooks or Story Flaws, which have been flavored with this particular personal flaw.