Can someone explain what a transmission service involves?

I have a 2007 ford fusion. Have had zero issues with it. Out of nowhere, about 3 weeks ago, I got in my car and out of nowhere the service light came on and it was tatted shifting Really hard. Took it to a friend, thinking it was the transmission. He checked the transmission fluid, it was black. Took it to a transmission shop, turns out it was an oxygen sensor, but they also did a "transmission service" because the fluid was so dark. 3 weeks later I take it for an oil change, fluids black again. No symptoms of anything wrong, no shifting hard… Just trans fluid that keeps getting dark. Any ideas?

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