What are those shadows on the edges?

I am currently modelling an horizontal stabilizer's joints. I am currently working on the holes in the moving part. I am using subdivision, and it shows black and white shadows around the external edges. I'd like to get rid of them while keeping a smooth model (ideally with subdivision on)?
I attached the subdivision settings and a better look at the geometry below. Ask me if you need more info/angles!

https://preview.redd.it/q97fkrlxl60f1.png?width=748&format=png&auto=webp&s=6b24f9627c325b508d383f82a9867f85ab79aba9

https://preview.redd.it/8r40or1xl60f1.png?width=427&format=png&auto=webp&s=62cbb60bbedc50c9b471abb4e3ca86a36c9214ed

https://preview.redd.it/jjuxptu1m60f1.png?width=499&format=png&auto=webp&s=c28d35234501471d5fb1df77249484affd6f9bca

https://preview.redd.it/u57jomw3m60f1.png?width=587&format=png&auto=webp&s=bac728c6167d9798034ecf80f25985dd929fd7f0