Black nail polish recommendations?

ILNP has a crème one-coater black called Ink. For me, the formulas in that collection can last up to 9 days with the right prep.

Prep really is essential. You could ask your wife to share her method, or alternatively, here's my routine:

• Using a strengthening remover, remove any nail polish already on your nails.
• Generously apply cuticle oil to your cuticles; make sure you really rub it in.
• Use a glass nail file to shape your nails and a glass cuticle stick to gently push back your cuticles. Germanikure makes really great products.
• Using 100% acetone and a q-tip or cotton ball, clean your nail bed. Avoid getting the acetone on your skin.
• If your nails are weak, consider using a nail strengthening treatment. If my nails become weak or start to peel, I use Rejuvacote 2. But if your nails are in good shape, you can skip this step.
• Apply one layer of base coat. I like Orly Bonder.
• Once your base coat is dry, apply your nail color. One-coater polishes can be applied in a single layer, neither thin nor thick. Other polishes should be applied in thin layers, waiting for each layer to dry before applying the next layer. Cap the end of each nail (i.e., be sure to paint the free edge of your nail, not just the top).
• Use acetone and an angled brush to immediately clean up any places where you paint "outside the lines".
• Apply a thick layer of top coat, again being sure to cap the nail. You'll want a quick-dry topcoat because otherwise you'll be waiting forever for it to dry and will probably still wake up with sheet imprints on your nails. I've been struggling to find a top coat I really love. Seche Vive is doing pretty well for me but the bottle is new, so we'll see if it gets gloopy halfway through. Make sure you get Seche Vive, not Seche Vite. Seche Vite dries super quick and has a nice finish, but it causes nail polish shrinkage which is annoying.
• After your polish has fully set and dried, apply cuticle oil again. I personally apply it after washing my hands every time, especially now that it's getting colder outside.

Lastly: no standard polish or top coat will save your nails if you're rough on them, so consider using gloves to wash dishes, don't open cans with your fingernails, etc. Another option is gel polish, which is practically indestructible, but most casual nail polish wearers don't apply or remove it themselves, they go to a salon. But it'll last you a few weeks, more or less depending on how fast your nails grow.

Good luck!