Hello, if you’re gifted with Photoshop, could I ask you a simple small question ?

In as little words as possible : how could I force a smooth transition between two zones of different colors, or of different grayscale tones/patterns ?

Content-aware and/or (more usually) the clone brush may help on small zones, but on large zones, I’m still lost for tricks.

Look at this example, I joined two pictures into one, and around the stockings of the girl, in the middle, there is an ugly vertical difference of tones.

Today as I write it, whatever I tried, I am unable to fix it. Clone brush ? Fail. Selecting the transition zone and content-aware ? Fail. Blur, sharpen, sponge, burn ? Fail.
– And I would really like to improve, so please, if there’s a way to do it correctly, I’ll be very grateful if you can share your knoweledge :)