source of Latent Magical Ability?

This idle speculation,

Just wandering if Mythic Blood might predispose a character to Latent Magical Ability, especially if the Mythic ancestor was not Hermetic.

The Mythic Blood might be trying to give the character other Supernatural abilities /virtues that the Hermetic parens was unfamiliar with. Or suppressed at Hermetic Opening due to insufficient Opening score. But the Mythic Blood won't let it go...

Totally up to you, but no real reason why you couldn't do it that way.

As I see Latent Magical Ability, it's more or less " a supernatural virtue that will be revealed at a later date". So why not pair it with Mythic blood?

I share that view and I never really used that virtue.

For that kind of stuff I usually just make the character have the virtue not knowing that he have it, if only because it saves time erasing pencil on the paper and makes character sheets live longer.

And now that I think of it, there are some cases when not knowing that you can do something isn't the same of not being able to do something. Or that having it coming at a later point can bypass things like these suppressed virtues Ivgreen said. Or have virtues that taints you as infernal or give you magical air not ruining your childhood because the virtue wasn't there back then. Which is interesting and suggesting, nice!

Latent Magical Ability is one of these story hookish virtues which loads the responsibility over on the SG. Therefore I have never used it nor encouraged it.

Like the story flaws, it is completely open-ended. Whatever fits your inspiration to make a good story goes, and a Mythic blood link is as good as anything.