Can I even use Devoted Blade if I'm Neutral?
Devoted Blade (Su)
Prerequisite: Magus 12
Benefit: Whenever the magus enhances his weapon using his arcane pool, he may spend 1 additional point from his arcane pool to add either the anarchic, axiomatic, holy, or unholy special ability to the list of available options. A magus may only add one of these abilities if it matches his own alignment.
Added for reference for you.
I would say that if your alignment as a magus is NN, then no you would not be able to use that Arcana at all. It specifically states you can only add said property if it matches a part of your alignment. Given that NN had no corresponding alignment portion, then I would say no-go.
Think of it this way from a non-mechanic purpose; the ability states Devoted Blade. If you are NN, then you are not devoted to anything when it comes to Order/Chaos or even Good/Evil. You follow your own path in that regards and are therefore not Devoted as per that arcana would suggest.
No, Devoted Blade last line reads "A magus may only add one of these abilities if it matches his own alignment." So a True Neutral character cannot use this Arcana.