Ditch GIMP and get Photoshop. From my experience GIMP very poorly handles image quality after each transformation (resize, rotate, etc). PS is much more powerful tool with millions of tutorials on YouTube.
Automatic GIMP blend options never get job done properly. I assume that's what you tried to achieve here. I suggest not to waste time on that. I did that at first, did not have a single success.
In your case I would try to darken and add yellow/red color to the BS skin to try to match HS before doing any blends.
Proper fakes are done using simple transformation, masking to hide and reveal areas and then color adjustments using RGB curves or selective color. YouTube helps a lot here.
Most importantly, it's difficult, maybe nearly impossible to make a good looking fake if angle, exposure, shadows or skin texture is not matching the body. Not to mention stupid photo lens filters or Instagram effects.
This is why I mostly do x-rays :uhuhuh-onion-head-emoticon: Much more easier if you find a body with good angle.
Also fake always looks much better if HS hair is used (good luck cutting out hair nicely ). That of course requires proper HS and BS.
I know this rambling of mine was not really helpful for your problem but I have been there so I thought I will share my experience.
first of all you have to reshape the BS face shape to be the same with the HS proportion... good fakes (in the fake point of view) always have good proportion.. with good proportion even if the color is a little bit off it will still looks good... perfect color sync but has bad proportion it will still bad.. sorry for my bad english
i use photoshop and this is my attempt
and if you aim for perfection on fakes just do hyper realistic digital painting or nude photograph... for fakes the time is not worth it to aim perfection such as shadow skin texture and shit, kept doing what are you doing dont listen to people who blabbering that and this fakes is fakes it will still be fake and in the original point of view it will never be perfect because its FAKE
let me know if you need photoshop guide... people in my channel will gladly help you