That might be his coloring. He has a black face, so his fins could be black too.
But it doesn't hurt to use some Aquarium salt in case he has a fin problem.
That and warmer water temperature, it REALLY helps.
My betta damaged his fin on the filter before, I switched one of the filters from another tank to his that stays around 82-83 degrees and used aquarium salt for 10 days...I already saw the difference in just a couple days.
If there is something else to use for fin rot, someone else may know!
And he should be fine, just do frequent water changes, and test his water with a test kit to see how the water quality is.
Depending on the results, it will let you know if you need to do any more water changes and how much.
We all make mistakes and learn from them, sometimes more than once!
But the point is learning and changing it,
you sound like you are doing your best to take care of him.
I learned alot from coming to this site, and still am!
Make sure the silk plants are not bugging him, that could be the problem too if there are any with his fins. Sounds crazy because you have silk plants, but sometimes they actually can have parts on them that is rough.
But you can take them out and sand them down if they are rough in some spots. This happened to me once before.