I would say it's possible Lucia isn't pure blood if it wasn't for the third eye she hides behind her circlet.

It could be that what ever prevents them from manipulating Aether is more an adaptation to an Aether poor environment. Or that people that live in Eorzea for prolonged periods can actually adapt to it and learn to manipulate ambient Aether from their environment like Conjerors do.

Im betting the later as "Aether Sickness" was actually a common, yet temporary, affliction some foreigners suffered when visiting Eorzea the first time and is originally blamed for our fainting spells prior to the echo's reveal. Especially in 1.0. From its description, dissyness, lightheadedness, fainting, possibly nausia and headaches (infured not stated) Aether Sickness is simmilar to Altitude Sickness and can be adapted too.

So Lucia's long presence combined with possible training may have given her some limited use of basic magic.