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?

Different, there's a filter change, filter and fluid flush, some newer cars say to back flush to clean filter out but it is better to not do that and change the filter and flush the correct direction.
I would not think your clutches are burning up if you are not having transmission problems so good chance they did not flush completely

Sometimes they will just drain the old fluid, replace it with new fluid, and replace the filter and gasket. Other times they will "flush" your transmission, which involves using new fluid to get rid of the old stuff. The advantage of the second way is that it gets rid of all the old transmission fluid, the first way leaves a lot of it behind, so that when you add new fluid, it just mixes with the old. That may be what's happening with your car. A lot of it has to do with how often you change the fluid (which you didn't mention in your question). Once a year is good, less if you drive a really low number of miles. So in your case, just get it flushed, and hopefully that will take care of it.

The transmission is burning the fluid due to clutches slipping. You need to get this back to a transmission shop to find out why this is slipping. If it isn't fixed soon, you won't have a transmission left.

Yeah, in the "old days" we had transmission dip sticks, all auto trans fluid was red and we were told to check perirodically and watch out for discolored or burnt smelling fluid.

My Fords haven't had dipsticks for many years - no way for the owner to check. My Fords are to be have trans service every 60,000 miles and I get it done at least that frequently. Mine have 2 filters that must be changed, and they get a real service: pans pulled, converter drained, new gaskets & proper fluid. I never have a cheap "flush".

So - we don't know if yours was serviced on time… If the filter(s) changed… Or what color new fluid of the correct type is. But as Stpaulguy sez, sounds like the clutches are starting to go - - not knowing how many miles are on the vehicle, service/use history, can't say if that's unusual or not.

Manufacturers have for a long time tried to keep mechanics honest by requiring them to remove the transmission oil pan to drain the oil. (i.e. No drain plug) Once the pan is off it is a simple process to replace the filter. It requires a little more labor but it is done right. Many shops are able to suck the oil out the fill tube and replace it with fresh. This technique does not drain the transmission well and does not replace the internal filter, nor does it drain the transmission cooler and converter. You probably got the cheap transmission service and have a quart or two of old burnt fluid mixed with the new and are still running fluid through and old filter. If you are simply servicing a good transmission you can get away with this, although I don't recommend it. It should never be done this way if there's burnt fluid and a suspected problem developing. Ask the previous servicing shop what they did in detail or look at what parts you were billed for. Look for "transmission pan gasket and filter" in the parts list.