No show and anti-blister socks?

Wright socks are double layer. They help to reduce or eliminate blisters. They are relatively expensive but worth it. They are very thin socks, by the way.

I don’t use socks in flats. But I do like removable insoles that I wash. I have some that are flat wool and some that are terry cloth. They don’t have any structure or support. But they do feel soft and absorb moisture. Easy to machine or hand wash and air dry.

Wool https://a.co/d/05CVMuzp these are black but I also have tan

Terry cloth https://a.co/d/0bEnucIs they come in white, tan or black

My Ondos are my favorite because they STAY ON but they are thin so I don't know what they'd be like for preventing blisters. They are comfortable. My feet don't do well with ballet flats but Ondo has ones that are no show with ballet flats. I recently bought a pack of similar ones (Frank & Oak) at Costco but I haven't worn them, yet. And, maybe they would show with some ballets, as they are like one style of Ondos, but not the skimpier design.
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I think Rothy's are meant to be worn without socks. I think you'd be hard-pressed to find any pair of socks that don't show in Rothy's, let alone socks that are specifically meant to prevent blisters (like the previously mentioned Wrightsocks, which I also endorse). I wouldn't be able to walk half a day in a ballet flat. Depending on where and when you're traveling, I would opt for a cute sneaker or Chelsea boot. For cooler weather, I used to have a pair of knee-high boots that were very comfortable and surprisingly compact. Sadly, they have fallen to bits.
Ballet flats are lightweight and easy to pack, though, so I often travel with them when I know I will be dressing up, but I only wear them to whatever the event is. For dinner in a restaurant or a show in a theater, I wouldn't bother. No one is looking at my feet.

I bought a pair of these Tioseban shoes on Amazon several years ago and they are perfect for dressier travel shoes. They're very lightweight, squash up smaller for packing, and look very dressy under pants and even a skirt. Plus you can wear just about any pair of no-show socks in them without them showing, due to the cut and design. When you wear them, it feels like you're walking on clouds—they are so comfortable and can also stand up to a decent amount of walking as well. In fact, I'm wearing them right now (I am subbing at a school today). https://www.amazon.com/dp/B092DJ121J

I think Lola was the one who first mentioned them. Here are a couple of other threads about them.
https://community.ricksteves.com/travel-forum/best-travel-shoes/tiosebon-shoes
https://community.ricksteves.com/travel-forum/best-travel-shoes/just-bought-another-pair-of-tiosebon-shoes-first-mentioned-by-lola

This the sort of thing you are looking for?
https://www.injinji.com/height/hidden.html

Bomba’s claims to have some. A couple of trips ago, I brought along my Rothy’s flats. I wore them one night to dinner and with their minimal support, my feet hurt after only a short walk over cobblestones to the restaurant. I can’t imagine wearing them for half a day of walking but they are cute!

https://bombas.com/products/womens-low-cut-no-show-socks?variant=beige&size=s&utm_content=socks+-+casual&country=us&utm_source=google&utm_medium=cpc&utm_campaign=NONBRANDSHOPPING-PLA_SSC_NB_NoShows-Sock-ACQ-NONE&gad_source=1&gad_campaignid=24191347846&gbraid=0AAAAADoMKpuC-ORBirVytHe15N6fWBsDN&gclid=EAIaIQobChMIjreI-ajilwMV183CBB21cQ8OEAQYASABEgJKyfD_BwE

I have several pair of SmartWool Low Cut Everyday No Show Socks. I found them on sale a few years ago and they work with my flats.

Thank you all! I love advice from people who know what they're talking about! 😀

Margie - that is sad news on Rothy's! So cute and full of promise!

Joe32F - argh, toe socks! I am sure they work for some but they give me the heebyjeebies! And it looks like they would show too.

CanAmCherie - I heard about the brand you recommend, I was just not desperate enough to pay the price, but I am getting there!

Mardee - I may try those especially that the consensus seems to be that even the best ballet flats are no good for long distance walks. Hard to resist the convenience of Amazon...

Jeanm- I ordered a pair of no show Smartwool socks, awaiting their arrival with trepidation! 😄

Thanks again and happy travels! 🧳

As opposed to dressing to impress strangers who in all likelihood aren't (and frankly oughtn't be) looking at my feet, I choose instead to travel as light as possible with only the footwear I trust from experience to not cause blisters.

Take your most comfortable walking shoes or similar, and get a pair in black. Done.